Someone debonair is charming or suave. They have good manners, and they probably look good too.
This French word has a fancy feel to it, and it is a fancy kind of word. Being debonair is a specific form of being charming that applies mostly to men. When you're debonair, you impress women, other men, and pretty much everyone with your manners, wit, and style. Debonair is a little bit of an old-fashioned word. Old movie stars like Cary Grant were often called debonair, but not many people are today.