A crest is a showy tuft of feathers on the head of a bird. It's easy to identify a male cardinal by its bright red color and the tufted crest on its head.
The Latin word crista, which crest comes from, means "tuft or comb." An animal's crest can be feathers, fur, skin, or a comb like a rooster's. Something that resembles such a feature can also be called a crest, such as a plume on an old-fashioned helmet. The top or highest part of something is also a crest, like the crest of a hill or the crest of a wave. As a verb, crest means "to reach the top" like when you crest a mountain.