Slender Man is a fictional character that was created on the internet to scare people. He is tall and skinny, with long arms and legs, and has no face. Some people say he can change his size or shape, and others say he can make people go crazy just by looking at them.
People like to tell scary stories about Slender Man, and some people even pretend that he's real. But don't worry, he's not really out there waiting to get you. He's just a made-up character for scary stories, like a monster in a movie or a TV show.
Just remember, if you get scared by something you see online or hear about from friends, it's always best to talk to a grown-up you trust to help you feel safe and calm.