To be plucky is to show courage. Plucky people are often underdogs fighting against the odds, like a plucky kid who scares away a burglar or a plucky kitten who refuses to run away from a Great Dane.
Pluck is courage or heart, so to be plucky is to have those qualities. This word describes brave people and actions, and it means about the same as feisty and spunky. It often applies to people who bravely struggle against powerful forces. A short person playing basketball is plucky. You can be plucky in fighting a terrible disease. Anyone who refuses to give up, no matter what, is plucky.