Daydreaming is when your mind wanders off to imagine things that you enjoy or think about things that you want to happen or that make you happy. It's like creating a story or a movie in your head, or going on an adventure in your imagination.
Imagine that you are sitting in a classroom, but instead of paying attention to the lesson, you start thinking about your favorite superhero and what powers you would have if you were a superhero too. Or maybe you start thinking about going to the beach and playing with your friends in the sand and waves.
Sometimes daydreaming can be a fun way to pass the time or to entertain yourself when you're bored. But it's important to remember that you should still listen to the person who is talking, or do your work, and not just daydream all the time.