Layers are a way to easily modify or change your graphic design. They make adding special effects to specific parts of the design very easy to achieve. Also you can delete a layer and not affect the rest of your design.
For this tutorial you'll need to have some basic gimp skills, like creating new documents, working with gradients, adding new layers, etc. We will not be going over these basic steps.
In this video we show you how to work a bit more with layers. using 3 photos, with a bit of cutting and pasting we then make it look like someone is coming or reaching out of the tv.