A champ is someone who wins a contest or a prize. The winner of an elementary school fifty-yard dash is a champ.
Champ is shorthand for champion — in other words, a winner or a victor. You might describe your favorite football team as a bunch of champs after they win the World Series. Another way to use the word champ is as a verb meaning "chomp," especially the way a horse bites nervously or eagerly at its bit. The original meaning of champ is "chew noisily," and the biting horse definition came next, in the 1600's.