Poop is a somewhat childish word for feces. When used as a verb, it means going number two.
Poop is a very commonly used slang word that means feces and is far more acceptable than crap and shit. The only downside is that talking about pooping or poop sounds kind of silly. Still, no one will blink if you say "I need to take a poop" or "Go clean up your dog's poop!" There are a few other unrelated meanings — like the rear "poop deck" of a ship and "the poop", meaning the inside information on something.