Imagine you're in a really cool playground with lots of fun toys to play with, including swings, slides, and a cool fort to climb. But suddenly, a mean kid comes along and decides to stand in front of the entrance to the fort so nobody can get in. That's kind of like what a denial of service attack does.
On the internet, there are lots of websites with fun things to do or important things to learn. But sometimes, bad people try to make those websites unavailable for everyone else by overwhelming them with requests. It's like sending a lot of people to the same slide all at once, so nobody else can use it.
When one computer tries to visit a website, it sends a request to a server (which is like the master of the playground). Normally, the server responds and lets the computer visit the website. But during a denial of service attack, lots of computers join together to send requests to the same server all at once. The server gets overwhelmed and can't keep up with all the requests. It's like too many kids trying to climb the fort at once, making it impossible for anyone to get in.
This can be a big problem because a lot of websites are really important. Banks, hospitals, and government websites all need to be available for people to use. Denial of service attacks try to make them unavailable, which can cause a lot of trouble for everyone. But luckily, there are smart people working to protect those websites and stop the mean kids from blocking the fort.