What do the little notes mean on Rock Band 2?

I know the whole game, but just not what those are. It’s when you’re playing guitar but instead of a big note, there are little notes that come up. Here’s an example:

http://wiimedia.gamespy.com/wii/image/article/941/941672/rock-band-2-20090105044134768-000.jpg

It’s the guitar on the far right and it’s the red note coming up