I think what they are refering to is a Water TRAP.

It should be in between your compressor & air line.
The best one I have found to use is a Toilet paper water trap that installs between you air compressor & air regulator.
Your local Auto Paint Jobber should know what I'm talking about.

You can also buy a plastic 2" water trap bubble that screws into the air regulator of your gun & then connect your air fitting to the bottom end & plug in air hose. Its a good thing incase you have water in your lines.
Water starts condensing in air lines in 25-30 feet of hose.

Spray