Complex sentence: GLSL shaders are not stand-alone applications; they require an application that utilizes the OpenGL API, which is available on many different platforms (e.g., GNU/Linux, Mac OS, Windows).
Split into simpler sentences: GLSL shaders are not stand-alone applications; they require an application that utilizes the OpenGL API. The OpenGL API implementation is available on many different platforms (e.g., GNU/Linux, Mac OS, Windows).