It means that you were really funny or that the person had a lot of fun with you. People might "hoot" (shout, go wild) at a great party.

Ameriwen is correct. I might also add that "hoot" is the word we use to describe the sound that an owl makes ... "Who! Who!" ... In fact the bird itself is sometimes called a "hoot owl". I am guessing that is how we came to refer to certain crowd noises as "hoots" -- loud sounds that have that long "oooo" sound, as in the word "who" -- those would be "hoots". If "you are a hoot", you elicit those sounds of approval from the people around you.