The motor shield implements one or more H-bridge motor controllers. Typically these will have two or three pins per motor. One pin controls the direction. One pin turns the power on and off and is usually attached to a PWM (analogWrite()) output pin so you can control speed. The optional third pin applies the brakes by shorting the two motor terminals for the voltage generated by the moving motor fights the motor and causes it to slow down faster.
Find documentation for your motor shield to find out which pins do what.
Reading the datasheet for the motor control chip on your motor shield can help in understanding.