When you tap into something or tap something you create an exit point for a resource of some kind. You are accessing that resource. This can be tapping a well, tapping a beer keg, stuff like that. It can also be used figuratively. For example: - I'd like to tap into this lucrative market.

"Tap the well" is not an English expression, as far as I can tell. Without further context, I can only give you an educated guess at the meaning. A well is the source of water (or some other desirable thing). When we tap into it with a faucet or tap, we allow its contents to flow to us. It is probably being used figuratively in whatever context you found it.

I think it's a slang expression for copulating / mating.