A secondary action is an action that occurs as a direct result of another. An example of this is a character bumping into something causing an object on that something to fall over, another example would be a charcter walking and swinging arms at the same time.
Secondary actions bring a scene to life and mimics what realistically both characters and objects can do.
An example of Secondary Action is this;
The horse is running, and the mane and tail moves in the same way as a result. Without the movement of the mane or tail, the physics of an animation comes into question as well as the environment of the character. Is it windy? Regardless of that, the movement of the tail adds personality to the character.