In computer graphics, a vertex shader is a program that is executed on the graphics processing unit (GPU) of a computer or game console. Its primary function is to manipulate the attributes of individual vertices in a 3D mesh, such as their position, color, and texture coordinates.

When rendering a 3D scene, the graphics pipeline first processes the vertices of the mesh, passing them through the vertex shader to perform any necessary transformations or calculations. These calculations can include operations such as translating, rotating, scaling, or deforming the mesh, as well as lighting calculations or texture mapping.

Vertex shaders are an essential component of modern 3D graphics programming, as they allow developers to create complex and realistic 3D scenes with efficient use of computing resources. By offloading vertex processing to the GPU, vertex shaders can significantly speed up the rendering process and allow for real-time graphics rendering in games and other interactive applications.

In Unity 3D, developers can create custom vertex shaders using the ShaderLab language, which allows for precise control over the appearance and behavior of 3D meshes in a game or application.