A vertex shader is a type of computer program used in graphics processing to handle the properties of individual vertices in 3D models. It takes input data, such as the position, color, and texture coordinates of each vertex, and transforms it into a format suitable for rendering. This transformation often includes operations like scaling, rotating, and projecting the vertices onto a 2D screen. Vertex shaders are a crucial part of the graphics pipeline, which also includes stages like fragment shaders and rasterization. They enable developers to create more complex and visually appealing graphics in applications such as video games and simulations by allowing for custom vertex processing.