The multiplying layers thing you're hearing about is how you get the textures on to your preset. Its just a way of creating layers on the initial sheet your working on so that the colors or patterns can show through even if you add a texture without just covering over your work with the texture. You can refer to this video here to understand how to apply the texture on to your preset since I believe that's what you're working on due to your post earlier
Start watching from 11:00 ish
http://www.youtube.com/watch?v=DEGw3-gbCyEHere's a screenshot I took of Gimp where you can see where the layer options thing is:
Click that little arrow beside the layers options and you'll see all the different options of layering
Goodluck and use the vid it's very helpful.
On the other hand if you're working on a map its pretty much the same process and you can multiply layers the same way.
