Someone who's jolly is extremely cheerful. Your jolly French teacher might spend a large part of each class laughing out loud at his own jokes — if only you understood French, you could laugh along with him.
Santa Claus might be the mythical figure best known for being jolly — he's especially famous for his jolly "Ho ho ho!" Anyone who tends to be merry or festive is jolly, and when you want to cheer up a less-than-jolly friend, you can jolly them, teasing or kidding until they smile. The Old French root is jolif, "festive, merry, or pretty."