"Dapper" is an old-fashioned word that means "handsome". You call a man "dapper" when he's clean, dressed nicely in a formal way, has a neat and stylish haircut, and so on.
Aside from calling a person "dapper", you can also say that a man's clothes are "dapper":
He wore a dapper white suit.
The word "dapper" sometimes appears in the combination "a dapper gentleman".