Secondary actions are like other actions that can be added to a main action, for instance let's say a character is reading the newspaper while toying with his breakfast (It could be cereals in a bowl). The toying bit is there to make the main action look more interesting, secondary actions are there to emphasize the main action or make it look more interesting.
I will use our group project to show secondary actions applied twice in the sames scene.
I-You can see a secondary action applied to the character when is in falling down underwater, his arms are swinging (since he is trying to getback to the surface, he is kind of swimming...), this is to emphasize the main action: The character falling down deeper and deeper.
II-Another secondary action applied when the character is swimming to pick the perl out of the shell, you can see that it is flapping its legs in the water to get to the shell and even when he picks it out of the shell, its legs are still flapping, it emphasizes the action and makes it look less dull.