As hunnybunny63 said, and as in the link that Bo B Bunny provided said, a binky is a way that rabbits express happiness. They jump into the air, sometimes kicking their back legs out. They usually also turn in mid air, and land facing the other way.
My rabbit also does what they explain as Half-Binkies, where he simply tosses his head as he's running, occasionally giving a little jump as he does so. Once you get used to it, it is very cute to watch some bunnies jumping and playing as they speed around while out on their run.
My bun, Ollie, does them very often to show he's happy. But don't be concerned if your bun doesn't binky: Some rabbits find other ways of expressing happiness, and some don't feel the need to tell you at all