Pin 5 is PWM output in the Arduino.
The PWM block from the Simulink Support Package for Arduino Hardware generates square pulses of varying duty cycle depending on the input value sent to the block on the Arduino hardware pin.
The value sent to the block input determines the width of the square wave, called duty-cycle, that the Arduino hardware outputs on the specified PWM pin. Here, the PWM output is generated at Pin 5.
Refer this documentation link for better understanding
