They have a safety device on the trigger. It amounts to a little plastic piece in the middle of the trigger. Pressing on the trigger from the front causes this piece to slide in, allowing the trigger to be depressed. It doesn’t do much but it would prevent the trigger from being pulled by something scraping it from the side, as might accidentally happen during holstering. It seems like the point of this device is to prevent accidental trigger pulls.

When you pull back the slide and chamber a round, you also pull back the striker. The gun is then ready to fire. The trigger pull is always the same weight. This doesn’t put the gun into an “extra light trigger pull” setting. After a shot is fired, when the gun cycles, this chambers a new round and pulls the striker back again. There’s no way to have a round chambered but not have the striker pulled back. Glocks don’t have a single action / double action like there is on some other guns, where the first trigger pull takes extra weight because it’s also cocking a hammer.