The brake system in a modern car is very complex, but when you break it all down they all work the same. The simplest description is that the brake pedal is depressed which compresses a piston in the master cylinder creating hydraulic pressure that sends fluid to each corner of the vehicle and activating the brakes. The system MUST be air tight and free of any contaminates or air bubbles. The sealed connection at each brake fitting is made by tightening a brake line and fitting into an opening that is shaped the opposite of the flare. Once tightened, an air and fluid tight connection is made.