GLSL, or OpenGL Shading Language, is a high-level programming language used for writing shaders, which are small programs that run on the GPU (Graphics Processing Unit). It allows developers to control the rendering pipeline in graphics applications, enabling effects like lighting, shadows, and textures. GLSL is designed to work with the OpenGL graphics API, making it easier to create visually rich applications, such as video games and simulations. By using GLSL, developers can optimize performance and achieve complex visual effects that enhance the overall user experience.