by default, the colour of any pixel on the screen is determined by its texture (basically, anyway). a shader lets you define the colour of each pixel's colour by functions on its texture, its normal, other textures, its position, etc. in addition, you can do the same with vertices.
normal mapping, screen space ambient occlusion, parallax mapping, bloom, depth of field, environment mapping, cell shading, and many other effects are done using shaders (look each up if you don't know what they are).
the best way to get started with them (if you have commercial or pro edition) is using the shader workshop (on the download page, or it may have come with a7 when you bought it depending on how recently that was).
julz
