Okay kiddo, have you ever heard of a bug? Not the kind that crawls around outside, but a bug in a computer. Just like how you need to wash your hands so you don't get sick, computers need to be protected from bugs so they don't get sick either.
BlueKeep is one of these bugs that can make a computer sick. It's a big bad bug that can crawl into computers running an old version of Windows, which is like the brain of the computer. It can make the computer act funny, like not letting the owner use it properly, or worse, someone else can take control of the computer from far away without the owner even knowing.
It's like having a stranger come into your bedroom while you're asleep and mess with your toys, except that stranger is actually someone who wants to use your computer to do bad things.
Luckily, grown-ups who work with computers are working hard to keep bad bugs like BlueKeep from crawling into computers. So if you have a computer, always make sure to keep it updated with the latest protection so the bad bugs can't get in.