The element of exaggeration and how it is important to your animation.

 Exaggeration in animation typically refers to a more pronounced action in facial expression or bodily movements that help to express a characters emotion, whether it be fear or love. Exaggeration may not be as dramatic as presumed either, the exaggeration  of expression can be as simple as

a slight upward turn of the mouth and a softening of the eyes of a character. Making them look happy in a gentle way with seemingly very little effort. Most of communication between people actually occurs in body language, so it is important for an animator to master hand gestures and a movement of the eyes and eyebrows so that people know what a character is experiencing.
Another object of importance when exaggerating is making sure the shots prior to your exaggerated scene are consistent to the characters perceived emotion. Like a quick shifting of the eyes and worried slope of the brow does not indicate that a character would break into a smile suddenly, more likely that a character would take on a panicked expression and perhaps run away from whatever they are facing.

If an animator is working on a more realistic series of expressions then their exaggeration would be more subtle like the above picture of the character Marinette from Miraculous Ladybug.