A facial expression conveys an emotion that tells us about the character and the way they react to the situation. It may also tell us something about that situation, eg if the character is very shocked when something happens. A facial expression can also convey the character’s true feelings. A character may be flattering another character verbally but a mocking eye-roll will show the audience the character’s true emotion. An actor’s facial expression may reveal a subtext or deeper meaning that contradicts what they say or do in the scene.
Emotions are feelings and a mood is the state someone is in, related to that feeling. "I feel happy. Therefore I am in a good mood." These are what facial expressions mostly reveal. Think about the different words available and try to use an interesting and precise word for what you want to express. Practise your facial expressions in front of the mirror so you can see how you interpret different emotions. Do you overact? Is your puzzled face too similar to your surprised face?