It means that the pistol or revolver hammer/striker is cocked as you pull the trigger then released at the end of the stroke. It takes more effort and a longer trigger pull to accomplish. Single Action would involve the hammer or striker already being cocked and the only effort required of the trigger is to release the sear which takes little effort or trigger travel.

James, double action means you have the same weight trigger pull and feel from the first through the last shot Sir.