What does a CO2 regulator do?

[Tijn_416cs]

I'm currently making an airsoft gun using somebrass attached to an air compressor pistol connected to an air/CO2 source.
I've been hearing about running regulated CO2 instead of an air compressor, however I don't know how it works.
It'd be awesome to make the gun skirmkishable, running on 12gram CO2 cartridges. The magazine on the gun will be a similiar system to a paintball hopper, where you just load bb's in from the top.
Tl;DR Making an airsoft gun, want it to run on 12g CO2 cartridges. heard about regulators, want to know more.
If you see any grammar mistakes, English is not my first language:3
construcive critisism is appreciated.