Okay kiddo, imagine you're watching a movie or reading a story. The person you think is going to be the main character, the hero or heroine, turns out not to be the hero after all. They're just pretending to be the hero, but really they're bad or selfish or mean. That person is called a false protagonist.
It's like when you think you're getting a piece of candy from your friend, but it turns out they were just pretending to be nice because they wanted something from you. They're not really a good friend, they're a false friend.
In stories, it's a way for the author to surprise you and keep you on your toes. You don't know who to trust or who is really going to be the hero. It's kind of like a game of pretend, but in a grown-up way.