Just like manual transmission cars, cars with automatic transmissions need a way to let the engine turn while the gears in the transmission and the wheels come to a stop. Manual transmission cars use a clutch, which completely disconnects the engine from the transmission when you depress the pedal. Automatic transmission cars use a torque converter. A torque converter is a type of fluid coupler, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, like when the car is idling at a stoplight, the amount of torque passing through the torque converter is very small, so keeping the car still
requires only a light pressure on the brake pedal.
If you were to step on the accelerator pedal while the car is stopped, you would have to press harder on the brake to keep the car from moving. This is because when you step on the accelerator pedal, the engine speeds up and pumps more fluid into the torque converter, causing more torque to be transmitted to the transmission and through to the wheels. When the internal parts reach a certain speed the Torque Converter will become a fluid coupler.