Specular reflection amount is the measure of how reflective a given material is. Any surface you look at, is somewhere on the spectrum between 100% reflection and 0% reflection.
At 100% would be surfaces like chrome, and mirrors.
At 0% something like dry plaster.
The more reflective a material, the more you see the reflection of the environment and the less you see the modeling factors (“light on form”). The more matte, non-reflective a surface is, the more clearly you see the modeling factors, while the reflection becomes gradually less visible.
You can create believable “semi-reflective” materials by blending a 100% reflective render with a 0% reflective (= matte) render. Think Photoshop Layers!
Important: the reflections themselves don’t change! They are in the layer on top. but you can change the material underneath.
Important: For additional realism, you will need to include the Fresnel Effect.