Lots of people think using a layer mask is only for the advanced Gimp users, fortunately this isn't the case at all. Let me explain you the basic usage of layer mask on a easy to follow step-by-step example.
Let me show you how to achieve that Depth Of Field effect known from the photography using Gimp. With other words, how to make selected area of the photo sharp and the rest out of focus.
Knowing how to select parts of your Gimp documents with the help of Quick Mask might be one of the first things you should learn when starting with Gimp. Let me introduce you to the basics of Quick Mask.