Positively is a word that places emphasis on another word or statement. A positively funny joke is an extremely funny joke.
Some words — like very and absolutely — exist mainly to turn the volume up on other things we say. Positively is like that. If your back hurts, you might say, "It's positively killing me!" If you want to see a movie, you might say, "I absolutely, positively must go." Someone might ask, "Are you positively sure you don't want some cake?" if they can't quite believe you don't want some. People use this word to emphasize a point.