Okay, imagine you like playing with LEGOs, and you want to build your own toy gun. Instead of buying a toy gun from the store, you want to make one yourself.
Now, let's say someone else has some special tools and materials that you don't have, and they offer to help you build a more advanced toy gun. You work together to make a gun that looks and works just like a real gun, except it's made out of plastic bricks.
This is kind of like the idea of a privately made firearm. It's a gun that someone builds themselves, either with their own tools and materials or with help from others. But instead of LEGO bricks, they use metal, plastic, or other materials that can be used to make a real gun.
Now, some people might build a gun like this just for fun or as a hobby. But other people might do it because they don't want to buy a gun from a store or go through the process of getting one legally. That's why some people have concerns about privately made firearms, because they can be harder to track and regulate than guns that are bought through official channels.