Okay kiddo, you know how when you light a candle the flame needs to touch the wick to start burning? Well, it's kinda similar with firearms. When you want to shoot a gun, you need to light the gunpowder inside the gun to make it explode and push the bullet out.
So, the touch hole is a little tiny hole in the side of a gun barrel that leads to where the gunpowder is stored. When you light a match or something and stick it in the touch hole, it lights the gunpowder and makes the magic happen.
Think of it like a secret doorway that leads to a big explosion! But remember, guns are dangerous and should only be used safely and responsibly by grownups who know what they're doing.