What is the button between the control button and the start botton on the keybourd?

I have always bin wondering.What is the button between the control button and the start botton on the keybourd?
If you hold down the Fn button and at the same time hit the up or down arrows your screen will become brighter or darker, any other keys with this same color (usually blue) will also do different functions play around a find outWhat is the button between the control button and the start botton on the keybourd?
Does it have a little Fn on it? its a function key, found on laptops. it allows you to use the features on select keys that are pictured in the same color as the Fn key's writing.
There is no button between the two.