Well, backbox is like a special type of computer tool that people use to do something called "penetration testing." That's when people try to find ways to break into a computer system to see if it is secure enough.
You know how sometimes you play hide and seek with your friends and you try to find them wherever they are hiding? It's kind of like that, but with computers. Backbox helps people find all the ways that someone could break into a computer system, so that people who make computers can make them stronger and harder to break into.
But it's important to remember that backbox is only supposed to be used for testing purposes, and you should never try to use it to break into someone else's computer without their permission. That would be very bad and could get you in a lot of trouble!