A fragment shader is a type of computer program used in graphics rendering, specifically in the OpenGL and DirectX frameworks. It processes data for individual pixels, determining their color and other attributes based on various inputs, such as textures and lighting. This allows for detailed visual effects in 3D graphics. Fragment shaders are executed on the GPU (Graphics Processing Unit) after the vertex shader stage. They play a crucial role in creating realistic images by applying effects like shadows, reflections, and textures. By manipulating pixel data, fragment shaders enhance the overall visual quality of video games and simulations.