Hammer ons and pull offs are notes that you can play without strumming as long as you hit a note that you have to strum before they come up. Hammer ons and pull offs are found in most songs on the hard and expert difficulties and are rarely found in medium or lower. They look like regular notes but with brighter, whiter circles in the center of them.
http://www.youtube.com/watch?v=oYchnqv8Vxo
From 2:16 - 2:18, those are Hammer ons.