There are two types of secondary action:
1. An action that is the result of another action.
2. Additional action aside from the primary action(s)
Secondary action helps add to realism. If you hit something it will
move. This is the same concept as, "for every action there is
an equal and opposite reaction". The reactionary actions must
be shown to create a authentic scene.
Additional action aside from the primary action can be many things.
It could be the waves in the ocean, that is in the background. An
animator must be careful when adding in this type of secondary action
because the eye is drawn toward movement and even the smallest movement
can draw away form the primary subject.