A fragment shader is a type of computer program used in graphics rendering, specifically in the OpenGL and DirectX frameworks. It runs on the GPU and is responsible for determining the color and other attributes of each pixel in a rendered image. By processing data from the 3D scene, it helps create realistic textures, lighting effects, and shadows, enhancing the visual quality of video games and applications. Fragment shaders work by taking input data, such as vertex colors and texture maps, and applying calculations to produce the final pixel color. This allows developers to create unique visual effects, like reflections or transparency, making scenes more immersive and visually appealing.