Can someone basically explain welding to me?

I'm working on a gun prototype for my new game, I notice on other people's weapons they have scripts involving welds. I was wondering if this affects the position of the gun and if someone could explain to me how they work. Thank you(:

Yes, a Weld needs a script to punch in a few values to it. Two of those values are references to the two parts being welded, so it knows which parts to weld. The other two values it needs are the positions of the same two parts (relative to each other?). Then a weld will make sure that the position of those two parts, relative to each other, will never change.

Use this: http://www.roblox.com/Weld-Plugin-item?id=148570182

Just select the parts you want welded and it welds them!