When you press the brake pedal, hydraulic fluid is sent from the master cylinder to the brakes at each wheel. The master cylinder has two ports (or openings) for fluid to flow out of. Each port is connected to a different brake line.
If there was only one port and one brake line, and that line became blocked or damaged, your brakes would not work at all. Having two separate lines means that if one line becomes blocked or damaged, you will still have some braking power because fluid can flow through the other line.