Hammer-On's are notes that you simply don't have to strum on.
Most notes are regularly notes that you would have to strum on, but some notes are close enough that you do not have to strum on them. You can mostly tell if they tips of the notes have a "black ring" around the white spot of the note. Hammer-On's do not have the "black ring" around the top of the note. Although this is for GH3! The way you tell on gh2 and gh1 is pretty much telling if the notes are close together or not.
A good song to practice for Hammer-On's is call "My Name is Jonas."
~Have Fun!