GLSL stands for OpenGL Shading Language. The GLSL shaders are available for use with python scripts. See the included example for a basic template to get started programming shaders for your game engine app. GLSL shaders come in two flavors, vertex and fragment programs. The vertex shader applies to per vertex operations. The fragment shader applies to per fragment(pixel) operations. Google is a good place to start for entry tutorials on the language. Shaders in the game engine are passed to blender in the form of python strings, and each material on a mesh can also represent a fully programmable shader.
The following pages in this document will serve as a reference to the interface with python.