A mood is an emotional state of mind. They can be positive or negative. People sometimes say they are in a "good mood", if they are happy, or a "bad mood" if they are sad, or angry. Moods usually last for several hours or days.
Sometimes, people's moods depend on what kind of personality they have. People who are optimistic are often in a good mood. People who are pessimistic are often in a bad mood. People's moods also change when things happen to them. For example, if something bad happens in a person's life, they may be sad or angry as a result.