Secondary Actions add to and enriches the main action of a character, enforcing the main action.
For example, a character who is angry walks with a forceful, aggressive walk. The secondary action would be the clenching of his firsts and any dialogue that accompanies the walk. This dialogue also effects how the head moves with the walk. All of these things are the secondary actions, however they cannot be too strong to distract from the main action. The all have to work in harmony together.
Images courtesy of giant-bean.
Video courtesy of AlanBeckerTutorials.