Which flower talks the most?
Q: Which flower talks the most?
A: Tulips, of course, because they have two lips!
This is a standard riddle in the Q and A format. The humor in this joke is a pun. The word Tulips, and the words Two Lips are pronounced exactly the same, and lips are required to talk, so between those two things, this is a fairly good pun.
It isn't really that funny as a joke, but this is joke explainer, and not funny joke explainer, so you are just going to have to deal with it.