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.
Simple yet very useful Gimp tutorial where we make a detail or an object in a Photo bigger and more visible by resizing it and adding a white rounded square frame around it.
If you found yourself in a position where you wanted to crop a certain photo into a perfect square but you just couldn't figure it out how, than this tutorial is for you.