Specifically, it's a tip left on the closed lid of your hotel room toilet that you have thoroughly clogged and left for the maid to clean up. In a broader usage, it's appropriate to use the term to describe any tip given to someone dealing with the dirty, smelly aftermath of your poop.
"I took such a big shit in my hotel room this morning I clogged the toilet and had to leave a poop tip for the maid." (Specific usage)
"The room service guy showed up right after I took a monster shit. The room smelled so bad I gave him a little bit extra as a poop tip just for walking in." (Broader usage)