A shader is the human-editable code that gets compiled into the GPU to do graphics rendering. In terms of 3D objects, "wireframe" is broken into vertices and fragments, where vertices are points in space and fragments are the interpolated positions between vertices. You pass any information you want to the shader from your main CPU process (e.g. time elapsed, screen resolution, textures you want to use, click events, random variables you conjured) and the shader executes whatever you code you wrote for it -- maybe it adds a blur effect, thresholds colors into black/white, mines bitcoin, it just executes whatever shader code you wrote, however creative you wanted to be. Some really cool examples at shadertoy.com