Okay kiddo, let me explain to you what a masthead rig is in a way that you can easily understand.
Do you know what a sailboat is? It's a boat that has a sail on it, right? And you know that the sail helps push the boat forward using the wind. Well, the masthead rig is the way the sail is set up on the sailboat.
In a masthead rig, the sail is attached to a mast that goes straight up in the air. The top of the sail is connected to the very top of the mast, which is called the masthead. The bottom of the sail is attached to a boom that goes across the lower part of the boat.
This way, when the wind blows, it fills up the sail and pushes the boat forward. The masthead rig is a very common way to set up sails on sailboats.
So there you have it, a masthead rig is just a way to set up the sail on a sailboat so that it catches the wind and moves the boat forward. Hope that helps!