SA is Single Action. Its the shorter lighter trigger pull.
DA is Double Action. Its the longer heavier trigger pull.
The way I understand it, after you rack the slide the gun is automatically in SA. Even if you engage the safety. IF the round does not fire (or you are dryfireing and don't rack the slide) you go to DA which will allow you a second attempt to fire the round. And its fairly natural to try to pull the trigger again.