U and V refer to the texture coordinates as they're applied to a 3D surface. Similar to X and Y axes, I suppose. Hence the term, "UV-mapping". And yes, Reflection_Value (and other value channels) do determine the strength of a given effect. You can plug nodes into these value channels to vary the strength from point to point. For instance, a greyscale image (like a transparency map) can be attached to Ambient_Value to make something glow with different intensities at different spots on the material, instead of one constant value across the entire material.