I really don't know how to describe it, but I'll try.

It's when you hammer your finger onto another fret without strumming. Pulloffs are pulling your finger off without strumming. Simple enough, but the hard part of explaining comes now.

You can only use them on parts of the song where the circles coming at you do not have a black ring on top. If you look for a black ring on top of all of the note circles coming at you, you'll realize that some do not have black rings on top. On the ones without the black rings, you can hammer on and pull off.