Mercury sable - brake release -?

Hi
i got a thing that says brake release in my ford - mercury sable 2004 car -
what is that thing for -
when does it need to be used

That is what is called a parking brake. It is used ONLY when the car is stopped and turned off. You pull or push the parking brake, you set it, and it sets the rear brakes, so that if your car for some reason, like you leave the car in gear or in neutral instead of park, then the parking brake will keep it from rolling.
But the parking brake is NOT, NOWHERE strong enough to stop your car when you are driving. YOu can certainly use it in case your reg brakes aren't working, but they will only slow you down a tiny bit. They're not made to stop the car when it is moving.

On the other hand, if you attempt to drive with the parking brake still set, it WILL create a big problem. You would be driving with rear brakes on, which would overheat the brakes, and then possibly catch fire, and then the car. So that is why there's a parking brake release, to be used before you begin moving.
You know if the parking brake is engage, because when you try to move, the car just is really sluggish, if not movable at all.

So it's good to use the parking brake when the car is parked. I ALWAYS use mine when I park my car, even just to run into the convenience store. Just a good habit, because I have forgotten to put the car into park! Stupid me, yeah, but at least I have a good habit to use the parking brake to cover that.