Many computer keyboards have an Fn key. It allows adding a second function to an existing key. An example is the F11 key. In combination with the Fn key it acts as a Volume Down:

Fn + F11 = Volume Down

Some other keyboards have this the opposite way – F11 acts as a Volume Down by default and you have to press and hold Fn in order to get the normal F11 key functionality.

If you want to change the default behaviour and you are lucky enough all it takes is to make the change in the BIOS of the computer. More likely your computer have no such setting so you need to use a keyboard re-mapping software. One such program is SharpKeys.