Their teachings are summed up...

Maha-Vakyam or "great sayings":--Tat twam asi (That thou art) and
Aham Brahmasmi (I am Brahman). This oneness of Soul and God lies
at the very root of all Vedic thought, and it is this dominant
ideal of the unity of all life and the oneness of Truth which
makes the study of the Upanishads especially beneficial at the
present moment.