The obverse is the main, or primary, side of something that has two sides. The obverse of a quarter is the side with the handsome profile of George Washington on it.
Obverse is very often used to mean "the front of a coin or medal," although you can also use it to describe the principal or more obvious side of anything with a front and a back. Confusingly, this word also means "the opposite," as in, "Your messy room is the obverse of your idea of yourself as a tidy person." The Latin root helps to explain this seeming conflict between meanings — obversus means both "directed toward" and "turned against."