The verb contrast means to show a difference, like photos that reveal how much weight someone lost by contrasting the "before" and "after" shots.
You probably know contrast in its relation to compare. To contrast something is to look for differences among two or more elements, but compare is to do the opposite, to look for similarities. It's easy to tell the difference if you remember that contrast comes from the Latin root contra, and means "against." Contrast is also a noun meaning basically the same thing — you might notice the contrast of a dark tree against a snow-covered hill.