What does double action mean on a co2 bb gun?

Double action in any gun means that when you pull the trigger it both cocks and releases the hammer and fires the gun all in one motion. Single action means you have to pull back the hammer and then release the trigger in 2 different single actions.

So Double Action moved two actions in one motion and

Single Action means you have to do two single actions to fire the gun each time in two separate motions.

Now think of a 45 auto. When you get ready to fire a real 45 auto you have to pull back the slide on the first bullet to get the bullet loaded and ready to fire. Then when you pull the trigger you fire the gun. ( This makes the first shot a Single Action movement, Two seperat things had to happen ) but after the first shot is fired the gun auto loads and cocks the gun for all subsequent shots. This makes makes it a Double Action movement, it is know as SA/DA.