Suppose you're driving down the freeway one day. As you're about to take the exit, you tap on the brakes -- and you don't slow down. No matter how hard you press, nothing happens. You have no brakes! What are you going to do?
When you press on the brake pedal in almost all cars today, you're pushing on a piston. That piston pushes on brake fluid in the master cylinder, pressurizing the brake fluid. It flows through thin pipes, called brake lines, to pistons at each wheel. Those pistons apply pressure to the brake pads, and they squeeze against a disk or a drum to stop the car. If you were to have a catastrophic loss of brake fluid or if someone were to cut your brake lines, nothing would happen when you hit the brake pedal.
The first thing to do if you ever find yourself in the "no brakes!" situation is to try pumping the brakes. If your brake lines have a small leak (instead of a cut), you may be able to pump enough fluid into the system to get things under control.
The next thing is to try the emergency brake -- this is definitely an emergency! If someone cut your brake lines, he or she was probably smart enough to cut the cable for the emergency brake as well, so let's say that you find that the emergency break is out of commission too.