A "melodrama" is a kind of story that has very dramatic, extreme events and emotions. Characters in a melodrama do things like
- fight to the death
- go insane
- fall very deeply in love
- scream at each other
Calling a real person "melodramatic" means that you think that they're acting too emotional. A "melodramatic" person acts like they're on stage in a play, even when they're living a normal life.
"Melodramatic" sounds negative. In fact, even calling someone "dramatic" sounds negative.