The OpenGL Shading Language (GLSL) is a high-level programming language used for writing shaders, which are small programs that run on the GPU. Shaders are essential for rendering graphics in real-time applications, such as video games and simulations, allowing developers to control the appearance of objects and effects in a scene. GLSL is designed to work with the OpenGL graphics API, providing a way to create custom visual effects and manipulate graphics data. It supports various types of shaders, including vertex shaders and fragment shaders, enabling developers to define how vertices are processed and how pixels are colored on the screen.