A chap is a guy or a fellow — a boy or man who's a friend, acquaintance, or a friendly stranger. You might ask a chap on the bus if the seat beside him is free.
You can refer to any male person as a chap, and you can also address him that way: "Hello there, old chap! I haven't seen you in ages!" It's more common in Britain than the US, where the chap will know what you mean but might look at you oddly. Chaps also means a cowboy's leather pants, and when it's a verb chap means to become dry, cracked, and painful from wind or cold.