Just like manual transmission cars, cars with automatic transmissions need a way to let the engine turn while the wheels and gears in the transmission come to a stop. Manual transmission cars use a clutch, which completely disconnects the engine from the transmission. Automatic transmission cars use a torque converter.
A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed 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 gas 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 gas, the engine speeds up and pumps more fluid into the torque converter, causing more torque to be transmitted to the wheels.