The underground spring that provides water to a well can be called a wellspring. You can also use wellspring to mean the plentiful source of things like good ideas or information.
If someone comments that you're a wellspring of jokes, they're using the word metaphorically, to mean that jokes seem to pour out of you like water from a spigot. Wellspring comes from the Old English welspryng, which had the literal "spring" meaning. When it's used figuratively, wellspring implies an infinite amount of something: "My dog is a wellspring of affection and stinky breath."