Ghost notes are beats which are played very quietly on a drum. They have two main purposes. First, they allow the drummer to keep time more evenly. Second, the quiet beats provide a backing tone to the music.

Take for example this pattern...

^     ^     ^       ^   ^ 
R L R L R L R L R L R L R L R L R

where the ^ characters indicate the accented beats, and the letters indicate the handing. All of the other beats are ghost beats.

This pattern provides a main rhythm usually counted as "1, 2, 3, 1 2" (not evenly spaced). The ghost notes provide a constant evenly spaced rhythm underneath.

This rhythm is a variation on the theme known as "3-2 clave" (3-2 after the main accented pattern), and is commonly used in samba and Cuban music.