This page is a tutorial by Hitchhiker about GLSL in OpenArena and how to use it. GLSL allows to use features of modern video cards to show "cool" graphic effects: a graphic card capable of Pixel Shader 2.0 features is required to use GLSL. If you are a player, just reading Manual/Graphic options#GLSL effects could be enough for you... if you are a mapper/developer who wants to create or apply GLSL effects (usually, for shaders to be used in maps), you should read this page.