This is a classic kśama or peace prayer. It comes from the Yajur Veda and is included in both the Isha and Brihadaranyaka Upanisads.
It is used to pacify and create a sense of acceptance and closure to a practice, acknowledging our imperfections and that even in our imperfections, more remains. We can settle ourselves in that ongoing wholeness.
It is also used as a teaching tool for the stories and concepts of Vedanta, and the relationship of our own self to the Great Self. I love these Upanishads as they feature Gārgī, the wife of a teacher, who proves herself to be an adept.
Oṃ pū॑rṇa॒mada॒ḥ pūrṇa॒mida॒ṃ
Pūrṇā॒t pūrṇa॒muda॒cyate |
Pūrṇa॒sya pūrṇa॒mādā॒ya
Pūrṇa॒mevāvaśi॒ṣyate ||
Oṃ śā॒ntiḥ śā॒ntiḥ śā॒ntiḥ |