Imagine having a toy airplane that you love to play with. Now, imagine you have a big car with a flat top roof. If you put your toy airplane on the flat top roof, it will look like your car has an airplane on it!
Merchant Aircraft Carriers, or MACs for short, are kind of like big toy cars with flat top roofs that are much, much bigger. They're giant ships that have a long flat surface on top where airplanes can take off and land, just like your toy airplane on that big car roof.
These ships were made in the olden days when there weren't a lot of big airports around. Instead of landing at an airport, the airplanes would land on the flat surface on top of the ship while it was in the ocean. This was very helpful during wartime, because the airplanes could land and take off from the safety of the ship while it moved around.
Today, merchant aircraft carriers aren't used as much as they used to be, but some countries still have them, and they can be very helpful for things like delivering supplies to places without airports or for doing important work at sea.