Handbrake is like the parking brake in a car. When you park your car on a hill, you pull up the parking brake so the car doesn't roll away. Similarly, when you use Handbrake on your computer, it helps you save video in a special way.
Imagine you have a movie on your computer and you want to make it smaller or save it in a different format so you can watch it on your phone. Handbrake can help you do that. It takes the big video file and makes it smaller by removing parts that you don’t need or by changing the way the video is stored.
Just like when you pull up the parking brake and your car stays still, using Handbrake means your video is saved in a way that is easy to watch on different devices, and it doesn't take up too much space on your computer or phone.