"Matinee idol" is a special name for a really popular movie star. They are usually really handsome or beautiful and lots of people love them. When these actors started getting big in the early days of movies, a lot of people would go to see their movies in the daytime- like during the afternoon. This was because those people were often called "matinee-goers" and they would go see the movies instead of doing other things during the day. So if you're a really famous actor and lots of people go see your movies during the daytime, you're a matinee idol!