An explosion is like when you blow up a balloon until it pops. When things explode, they blow up really fast and make a loud noise. It happens because there is a lot of pressure inside something and it can't handle it anymore.
Imagine you have a tiny toy car that you put in a box and shake it around a lot. The car would bounce around and could crash into the sides of the box. Now imagine if you put lots and lots of tiny toy cars in the box and shook it really hard. Eventually, the cars might start hitting each other very fast and hard, and the box might break or explode!
Explosions can be caused by many things, like chemicals that react with each other, or by too much heat or pressure building up inside something. When they happen, they can be very dangerous, so it's important to stay away and let the experts handle it.