Okay kiddo, imagine you have a big truck and you use it to carry toys and snacks to your friends' houses.
Now imagine that the truck is a really big boat that carries things like cars, clothes, electronics, and even food across the ocean to different countries.
Sometimes bad guys called pirates try to steal the things on the boat, because they want them for themselves. That's where an armed merchant ship comes in.
An armed merchant ship is a boat that has special guns and weapons on it, kind of like your toy nerf guns but much bigger, that help protect the boat and the things on it from the pirates.
Just like you wouldn't let someone take your toys or snacks, the people on the armed merchant ship use their guns and weapons to scare the bad guys away and keep everyone safe.