Pabhassaramidaṃ, bhikkhave, cittaṃ. Tañca kho āgantukehi upakkilesehi upakkiliṭṭham. Taṃ assutavā puthujjano yathābhūtaṃ nappajānāti. Tasmā ‘assutavato puthujjanassa cittabhāvanā natthī’ti vadāmī”ti.
Pabhassaram—this could be translated as ‘bright’, and it should be regarded as something lit up, shining, effulgent or lucid. To find out how this term should be understood, one can bring to mind the term ākāsa—space—which literally means ‘shining forth’, in the sense of a space which is lit up. In MN 49, we can see that consciousness is referred to as viññāṇaṃ anidassanaṃ anantaṃ sabbato pabhaṃ, where ‘non-indicative’, ‘infinite’ (or endless, limitless) and ‘completely radiant’ (i.e. utterly and fundamentally lucid, not to be understood in a sense of mystically beaming rays of light or similar), are the terms referring to viññāṇa.  The whole point is that ‘brightness’ is an epithet describing the inherently infinite structure of the mind, which is ‘defiled’ by the obstructions (i.e. made finite). In SN 35.202 the Buddha said:
How, friends, does one not leak? Here, friends, having seen form with eye, a monk does not incline towards pleasing forms, he is not affected by displeasing forms. He lives aware of the body there and with mind that is boundless.
A monk can dwell and live his life with a “boundless mind”.   He has understood the nature of the body there together with the arisen structure of the present experience,  which is infinite. By understanding the infinite or boundless, any boundaries and finiteness that were present as a result of that lack of understanding, are removed. In other words, the ‘brightness’ or ‘lucidity’ or ‘infinity’ or ‘boundlessness’ are the ways of describing the property of the fundamental structure of experience, which is the very reason why overcoming the obstructions is possible.
Āgantuka—here it is translated as ‘superimposed’. It can also mean adventitious, incidental, accessory. The main point that has to be borne in mind is that the ‘obstructions’ are “incidental”, in the sense that they are not caused by the pabhassaramidaṃ cittaṃ, but are nevertheless there. The defiling obstructions do not have anything in common with the brightness of the mind; they are simply “layered” there, superimposed without actually affecting the infinite structure. Thus, since the infinite mind does not need those obstructions, it can be rightly said that they are completely gratuitous. 
So, obstructions, or more precisely avijjā, have no first point when they are manifested before which mind was pure, bright and unpolluted. Because of this it is not correct to refer to ‘bright mind’ as an original thing, as if it existed in a pure state before the obstructions arrived. Since there is no beginning to the infinity, nor to the beginningless avijjā, the notions of “first this, then that” have no place to apply. So how shall this evident priority of the ‘bright mind’ be understood then? The answer is straight—in an atemporal, structural sense. The infinity of the mind precedes avijjā ontologically, so to speak, while they are there both, simultaneously present. This should also shed some light onto why the structure of the mind does not imply nor require the obstructions (hence they are called ‘obstructions’)—they are superfluous, unnecessary, superimposed. Yet, the uppakilesā are there, and are not to be found elsewhere.  To put this all in different words: the infinity of one’s mind is atemporally more fundamental than one’s own nescience in regard to it. Furthermore, it is because of the nescience in regard to the nature of infinity, that avijjā is a beginningless phenomenon. If infinity has no beginning, how could the fundamental ignorance of that be different?
This is not all, there are further implications of one’s ignorance of the infinite structure—namely, permanence. The uninstructed worldling confuses the notion of infinity (no beginning, no end) with the notion of eternity, or rather identifies the two. In this way, a present experience is regarded as eternal or permanent. What a puthujjana does not see is that eternity implies infinity, but the infinity does not imply eternity.  Because of the nescience in regard to this, the puthujjana’s experience of infinity assumes permanence. In other words, his citta is regarded as permanent; he regards his Self as permanent. If one would be able to see the infinity without eternity, or even to see it as impermanent, the notion of Self, and everything else that depended upon it, (which required the notion of permanence), would cease without a remainder. Knowing infinity as something present (i.e. arisen) but impermanent (for the very reason that it has arisen on its own accord), clears the mind of any obstructions, any superimposed interferences with the infinite structure. Hence it is said that the arahant’s mind is immeasurable.  His citta has been developed to the structurally more fundamental state, and it is because this state was always possible (but not actual)  that transcending the obstructions was feasible. In other words, if one’s mind was inherently ignorant—i.e. avijjā structurally preceded any experience—arahantship would not be possible. (See footnote  above). 
Bhikkhu Ninoslav Ñāṇamoli
1. Ñāṇavīra Thera took sabbatopabhaṃ to be sabbato-apahaṃ (from apahoti, a+pahoti), in which case this compound would be synonymous with anidassanaṃ, ‘non-indicative’ or ‘not-producing'(Self). (Cf. Ñāṇavīra Thera—Clearing the Path (2010), ‘Note on Paṭiccasamuppāda’, footnote j, p.28). Whether he was right or wrong in reading it like this, it is clear here that either way the context is scarcely affected—the infinity of consciousness does not indicate Self, does not produce Self, and it is fundamentally lucid, radiant, and unobstructed.
2. Cf. MN 38.
3. This also indicates that the states of ‘boundlessness’, ‘infinity’ and similar that are mentioned in the Suttas are not exclusively reserved for “meditation attainments”, as they are commonly regarded.
4. Cf. “There is just this body and name-and-form externally…”—SN 12.19.
Craving, however, is a gratuitous (though beginningless) parasite on the intentional structure described here, and its necessity is not to be deduced from the necessity of intention in all experience. Intention does not imply craving—a hard thing to understand! But if intention did imply craving, arahatta would be out of the question, and there would be no escape.
—Ñāṇavīra Thera, op. cit., footnote f, p. 56.
6. Cf. MN 44.
7. The reader might notice that Ñāṇavīra Thera in ‘Fundamental Structure’ (op.cit. p. 107) refers to the endurance of a thing as ‘eternal’. This structural ‘eternity’ is what is meant here by ‘infinity’. It is just how one decides to designate these terms. If we were to choose the term ‘eternal’ to describe the property of the structure, we would additionally have to qualify it by saying: “A thing endures for ever. A thing is eternal… until it ceases.” I, personally, prefer ‘infinite’ so that the distinction between the inherent nature of the arisen experience (infinity) becomes apparent from the puthujjana‘s appropriation of it (eternity).
8. Cf. AN 3.115 & SN 35.202.
9. Cf. nibbānadhātu
10. For these very reasons the Buddha was able to “rediscover” the path which lay there. For more cf. SN 12.65.