Along with the Brihadaranyaka Upanishad, the Chandogya Upanishad is perhaps the oldest of the Upanishads. The Upanishads make up the literary form that completes the sacred literature known as the Vedas. They express the general philosophical and theological positions that are implicit within the earlier Vedic literature.
The Vedas are divided into four groupings, the Rigveda, the Samaveda, the Yajurveda, and the Atharvaveda. Each of these are associated with Vedic schools which preserved the literature. Some of these schools also preserved Upanishads that express ideas consonant with their pariticular Veda. The Tandins preserved the Samaveda and the associated Chandogya Upanishad.
Perhaps the most famous passage from the Chandogya Upanishad is "that art thou." This expresses one of the most significant ideas in all of the Vedas, that there is no distinction between subject and object. The Chandogya Upanishad clearly stakes out an answer to the problem of the one and the many which asks whether the universe is most fundamentally a unity or a plurality. Not only does the Chandogya Upanishad come down on the side of monism, it identifies the substance of that monism as mind.
Of course, there is a sense that the mind is separate from the external world and that it is an individual among other minds; however, the truth, according to the Chandogya Upanishad is that this is an illusion masking reality. Famously, that reality is betokened by the word or symbol "om."
The content of the Chandogya Upanishad is similar to the Brihadaranyaka Upanishad. It is, however, less cohesive and direct than the Brihadaranyaka Upanishad and seems to emphasize the esoteric, apostolic character of the Upanishads. The word "upanishad" is often translated two ways: first, as "secret knowledge," but also as something like "to sit at the feet of." The Chandogya Upanishad contains numerous stories of secret knowledge being passed on from teacher to pupil and in this respect is a literary enactment of upanishadic teaching.