Passing a variable to an OpenGL GLSL shader

I'm writing an iPhone app which uses GLSL shaders to perform transformations on textures, but one point that I'm having a little hard time with is passing variables to my GLSL shader.

I've read that it's possible to have a shader read part of the OpenGL state (and I would only need read-only access to this variable), but I'm not sure how that exchange would happen.

In short, I'm trying to get a float value created outside of a fragment shader to be accessible to the fragment shader (whether it's passed in or read from inside the shader).

Thanks for any help/pointers you can provide, it's very appreciated!