Verily Dadhyak Atharvana proclaimed this honey to the two Asvins, and a Rishi, seeing this, said (Rv. VI, 47, 18): 'He (the Lord) became like unto every form, and this is meant to reveal the (true) form of him (the Atman). Indra (the Lord) appears multiform through the Mayas (appearances), for his horses (senses) are yoked, hundreds and ten.' This (Atman) is the horses, this (Atman) is the ten, and the thousands, many and endless. This is the Brahman, without cause and without effect, without anything inside or outside; this Self is Brahman, omnipresent and omniscient. This is the teaching (of the Upanishads).

In plain words: The Lord takes on every form, and appears as many through maya, meaning appearances, working through the countless senses like a chariot pulled by many horses. Yet underneath, there is only one Brahman, with no cause, no result, no inside and no outside. This Self is Brahman, present everywhere and knowing everything; this is the core teaching.