Question about controlling DC motors

I found a code online to control two DC motors using an H bridge, and I wanted to know if someone could explain part of the code to me.

Why does i=0 set at HIGH make the motors go forward, but i=1 set at HIGH makes the motors go backwards.

Thanks!

// Use this code to test your motor with the Arduino board:

// if you need PWM, just use the PWM outputs on the Arduino
// and instead of digitalWrite, you should use the analogWrite command

// —————————————————————————  Motors
int motor_left[] = {2, 3};
int motor_right[] = {7, 8};
int ledPin =  13;    // LED connected to digital pin 13

// ————————————————————————— Setup
void setup() {
Serial.begin(9600);

// Setup motors
int i;
for(i = 0; i < 2; i++){
pinMode(motor_left[i], OUTPUT);
pinMode(motor_right[i], OUTPUT);
pinMode(ledPin, OUTPUT);
}

}

// ————————————————————————— Loop
void loop() {

drive_forward();
delay(1000);
motor_stop();
Serial.println("1");

drive_backward();
delay(1000);
motor_stop();
Serial.println("2");

turn_left();
delay(1000);
motor_stop();
Serial.println("3");

turn_right();
delay(1000);
motor_stop();
Serial.println("4");

motor_stop();
delay(1000);
motor_stop();
Serial.println("5");

digitalWrite(ledPin, HIGH);   // set the LED on
 delay(1000);                  // wait for a second
 digitalWrite(ledPin, LOW);    // set the LED off
 delay(1000);                  // wait for a second

}

// ————————————————————————— Drive

void motor_stop(){
digitalWrite(motor_left[0], LOW);
digitalWrite(motor_left[1], LOW);

digitalWrite(motor_right[0], LOW);
digitalWrite(motor_right[1], LOW);
delay(25);
}

void drive_forward(){
digitalWrite(motor_left[0], HIGH);
digitalWrite(motor_left[1], LOW);

digitalWrite(motor_right[0], HIGH);
digitalWrite(motor_right[1], LOW);
}

void drive_backward(){
digitalWrite(motor_left[0], LOW);
digitalWrite(motor_left[1], HIGH);

digitalWrite(motor_right[0], LOW);
digitalWrite(motor_right[1], HIGH);
}

void turn_left(){
digitalWrite(motor_left[0], LOW);
digitalWrite(motor_left[1], HIGH);

digitalWrite(motor_right[0], HIGH);
digitalWrite(motor_right[1], LOW);
}

void turn_right(){
digitalWrite(motor_left[0], HIGH);
digitalWrite(motor_left[1], LOW);

digitalWrite(motor_right[0], LOW);
digitalWrite(motor_right[1], HIGH);
} /

Because motor_left[0] and motor_left[1] refer to pins 2 and 3. Setting pin 2 HIGH makes the motor go forward. Setting pin 3 HIGH makes it go backwards.

The same holds true for the right side motor.