rear shock query

[car.crash]

i removed my rear shock today and i can adjust the spring to wind up or down, but at the rear of the shock there is a type of a valve like a tyre valve. what does this do?

 

[Mack]

Allows you to pump gas into the shock.

 

[tubes380]

mate i adjusted the spring today on my pitpro put the collar down the thread to stiffen the whole setup it made such a difference the bike sits higher when you sitting on it now and its alot more confidence inspiring

also my shock says that 0.5Mpa of nitrogen needs t be put into the shock every other month what does this mean and where am i supposed to get it filled?