A sire is an animal's father. A newborn foal might look very much like his sire, with a white stripe on his brown face.
Every once in a while, the word sire is used for a human — your dad is your sire — but it's much more common to find this word describing an animal's male parent. It's also a verb, meaning "to father," as in "My prize pig sires the cutest piglets." In the old days, you'd also use sire to directly address a nobleman or a king. It comes from the Latin word senior, "elder."