Vertex attributes offer one avenue for providing input to shaders; a second technique is uniform variables. Uniform variables are intended to be used for data that may change relatively infrequently compared to per-vertex attributes. In fact, it is simply not possible to set per-vertex attributes with uniform variables. For example, uniform variables are well-suited to the matrices used for modeling, viewing, and projective transformations.
Within a shader, uniform variables are read-only. Their values can only be changed from outside the shader, via the OpenGL API. However, they can be initialized within the shader by assigning them to a constant value along with the declaration.
Uniform variables can appear in any shader within a shader program, and are always used as input variables. They can be declared in one or more shaders...
