Okay kiddo, so imagine you are playing with toy boats in a big pool. Some of these boats might have words or letters on them, like "SS" or "HMS". These words or letters at the beginning of the boat's name are called a ship prefix, which is just a fancy way of saying a special name you give to a boat.
Different countries have different ship prefixes that they use for their boats. For example, the United States uses "USS" for their navy boats, while the United Kingdom uses "HMS" for theirs. These prefixes help people know where the boat is from and what kind of boat it is.
Now imagine you are playing with lots of bigger boats in a really big ocean. Sometimes, these boats need to talk to each other to help keep each other safe. They use radio communication to talk to each other, and one of the first things they say is their ship prefix. This helps them know who they are talking to, and what kind of boat they are talking to.
So in short, a ship prefix is just a special name that helps people identify a boat and where it comes from, and it's used to talk to other boats on the ocean.