Watch the Forearm Muting online guitar lesson by Stu Hamm from Stu Hamm U: Slap Bass

When you learn to use your forearm to mute the strings when you are playing with your thumb, you will be able to add a whole new level of dynamics and different tones to your playing.

In a rock music context using the thumb can generate a pick-like sound, and you are going to need to find ways to change the tone and volume. All of this can be accomplished by the amount of pressure you apply to the strings near the bridge with your forearm.