Today, after 3 hours of game, I gained
Quick Load - Successfully perform a fast reload.
but I really didn't understand what it is and how I got it. Can you explain me what to "perform a fast reload" means?
When reloading a gun, there's a meter in the bottom left corner. Inside the meter there is a small grey rectangle. If you press your reload key a second time while the white bar is inside the grey rectangle, you will reload in double quick time (and work towards the relevant achievements).
You may find it helps to move the reload bar much closer to your character by changing the following cvar:
You'll have come across this before if you've ever played Gears of War.