A pixel shader, also known as a fragment shader, is a program that runs on the graphics processing unit (GPU) of a computer or game console. Its primary function is to determine the color and other attributes of individual pixels in a rendered 3D scene.

Pixel shaders work in conjunction with vertex shaders to produce the final image that is displayed on the screen. After the vertex shader has processed the vertices of a 3D mesh, the pixel shader takes over, determining the final color and other properties of each individual pixel based on the lighting, texture mapping, and other visual effects applied to the mesh.

Pixel shaders are a critical component of modern 3D graphics programming, as they allow developers to create complex and realistic visual effects in real time. By offloading pixel processing to the GPU, pixel shaders can significantly speed up the rendering process and allow for more advanced graphics effects, such as reflections, shadows, and refraction.

In Unity 3D, developers can create custom pixel shaders using the ShaderLab language, which allows for precise control over the appearance and behavior of individual pixels in a game or application. Pixel shaders can be used to create a wide range of visual effects, from simple color adjustments to complex lighting models and 3D texture mapping.