Kung maputulan ka nito, hindi gagana ang lahat ng brake system mo at tatagas lang ang mga brake fluids mo palabas, which inturn you will lose braking power on all four wheels.
Nowadays, even if you have a torn brake line, you will still have braking power left because the hydraulic system of the LF tire and the RR tire are independent from hydraulic system of the RF tire and the LR tire. On other vehicles, the front brakes are independent of the rear brakes.
Brake lines are the rubber hoses that enter into the brake calipers. Kitang-kita sila from underneath or when the wheels are off.