Chandogya Upanishad is a Sanskrit text that is part of the ancient Indian collection of spiritual texts known as the Upanishads. It is considered one of the oldest Upanishads and is part of the Sama Veda.
Verse 5.10.7 of the Chandogya Upanishad is part of the section known as the “Ten Commands” (Daśa-Vakya) which contains a collection of spiritual teachings.
The specific verse you are asking about, 5.10.7, reads as follows in translation:
“This person who has been known as the Self, smaller than a grain of rice, smaller than a grain of barley, smaller than a grain of mustard seed, smaller than a grain of millet—this person within the heart is the self, and that is what must be sought after, that is what must be realized.”
This verse is pointing towards the idea that the true self or the atman is present within us, and it is small, subtle and hidden. The verse is emphasizing that the atman is present within the heart, and it is what must be sought after and realized. The verse is emphasizing that true self is not the physical body but something that is subtle and hidden. The verse is also pointing towards the idea of the importance of self-realization and self-knowledge in order to attain liberation and union with the ultimate reality.