I think it has to do with something she does when she's on top, but I'm not too sure.
I think its speaking about the band's crazy sex episodes with random girld from their past.
So that says he wasn't in a committed relationship with his ex and it was solely about desire.
But what makes the song so sad is that the protagonist has lost his way.
This Slang page is designed to explain what the meaning of make it rain is.
When You See Videos of Children With Cancer, How Does It Make You Feel? See more words with the same meaning: the shower.