Okay, so imagine you have a bunch of toys in one basket and your friend has a bunch of toys in another basket. But, you both want to have more toys than the other person. So, you start taking some of your friend's toys and putting them in your basket. But, your friend doesn't want you to have more toys than him, so he starts taking some of your toys and putting them in his basket.
This is kind of like what happened during the tonnage war. There were two sides, the Allies (which included countries like the United States, Great Britain, and France) and the Axis (which included countries like Germany, Italy, and Japan). Both sides wanted to have more ships than the other side because ships were really important for things like transporting troops and supplies during war.
So, they started building more and more ships to try to outdo each other. This was called the tonnage war because it was all about who had more tonnage (which is basically a fancy way of saying who had more ships and how big they were).
But, building ships takes a lot of time, money, and resources. And, sometimes ships would get sunk during battles or by things like mines. So, both sides were always trying to build more ships than the other side to make up for any losses they suffered.
In the end, the Allies were able to build more ships and have more tonnage than the Axis powers, which helped them win the war.