Okay, so imagine you're playing a game of hide-and-seek with your friends. You want to be able to catch them without them knowing you're there, right? That's kind of what the Mary B Mitchell was like.
The Mary B Mitchell was a secret ship that the British used in World War I to try and catch German submarines. They pretended to be a harmless, innocent ship that the Germans wouldn't suspect was a threat. But really, they were armed and ready to attack any German submarines that came near.
It's kind of like pretending to be a regular kid during hide-and-seek, but really you're hiding with a water gun and ready to surprise your friends. Only instead of a water gun, the Mary B Mitchell had big guns and torpedoes.
The Mary B Mitchell was called a "Q-ship" because Q was the code name for the British department that was in charge of secret weapons and technology. So basically, it was a sneaky, disguised ship that was really a weapon against the enemy.
Does that make sense, little one?