Sat-chit-ananda, being, consciousness, bliss, is the standard formula for the nature of Brahman in Vedanta, and particularly in Advaita. On the Advaitin reading the three terms are not attributes qualifying an underlying substance, since a Brahman with real attributes would be composite and so not ultimate; they are the one reality indicated in three ways. They function by pointing rather than by classifying, and the tradition is explicit that they are not a definition in the ordinary sense.
The components appear in the Upanishads before the compound does. Taittiriya 2.1 defines Brahman as satyam jnanam anantam, reality, knowledge, infinite. The Brihadaranyaka describes the self as consisting of consciousness. And the Taittiriya's analysis of the five sheaths culminates in the anandamaya, the sheath of bliss, with the third chapter identifying Brahman as bliss. The compound itself is later, standard in Vedantic and Puranic literature by the medieval period.
Each term does polemical work against a rival account. Sat, being, asserts that the ultimate is real, against readings of the absolute as a void or a mere negation, a point pressed in Vedantic polemic against Buddhist sunyata, which Buddhist writers reply misdescribes their position, since emptiness is the absence of inherent existence and not the absence of everything. Chit, consciousness, asserts that the ultimate is aware rather than inert, distinguishing it from the unconscious prakriti of Samkhya, which is real but does not know. Ananda, bliss, asserts that it is not neutral but is the fullness that desire has been looking for all along, and grounds the practical claim that realisation is not the achievement of indifference.
The devotional traditions take ananda in a fuller sense still. For Gaudiya Vaishnavism, Krishna is sac-cid-ananda-vigraha, the form composed of being, consciousness and bliss, so that the formula describes a person with a body, qualities and relations rather than an impersonal absolute. Whether the ultimate has form and personality is precisely what the Vedanta schools disagree about, and the formula is a piece of common ground that each of them reads in its own way, which is a good illustration of how shared vocabulary in Indian philosophy can conceal rather than reveal the extent of disagreement.