Okay kiddo, have you ever seen fireworks? When they explode, the sparks and colors spread out in all directions, right? Well, imagine if the explosion was much bigger and had something like a bomb inside of it.
The blast radius is how far the explosion can reach and cause damage. It's like a big bubble around the bomb that shows where the explosion can hurt things. Outside of that bubble, things might still get knocked over or jostled, but they won't be seriously hurt.
And just like with fireworks, the closer you are to the blast, the more damage it can do. That's why it's really important to stay away from explosions and let the experts handle dangerous things like bombs.