Hey kiddo, do you like pirates? Well, a merchant raider is kind of like a pirate ship from a long time ago. Except, instead of just stealing treasure and rum from other ships, they were kind of like sneaky spies.
The raider ship would pretend to be a regular merchant ship, just carrying regular stuff like food and clothes. But really, they were secretly carrying weapons and their job was to attack other ships that were enemies of their country, without anyone knowing it was them.
So they would sneak up on other ships, quickly attack them, and then sail away before anyone knew what was happening. It was kind of like a secret mission to sabotage the enemy.
However, this type of fighting was not really fair and many wars have rules against it. Nowadays, we don't really have merchant raiders anymore, but we still have special forces who do secret missions, just like them.