When a photographer zooms in on your face with her camera, she's taking a closeup. Without closeups in movies, you wouldn't be able to get a good look at the actors' expressions.
A closeup is exactly what it sounds like: a photo or video that gets up close to its subject. A film director might say, "Okay, now I want a closeup of his face when they say goodbye for the final time." Or your grandma might ask you to send her a closeup photo of your cat, so she can get a good look at just how long his whiskers are.