int led = 9;  //Defines, or sets the name led to be used in pin 9  
int brightness = 0;  //Variable that sets the brightness of the LED
int fadeAmount = 5;   //Variable that sets the rate the light will change, or brighten/dim

void setup() {
  pinMode(led, OUTPUT);  //Sets the the pin that is defined by led as an output
}
void loop() {
  analogWrite(led, brightness);  //Writes (sends) the varying brightness value assigned to the LED
   brightness = brightness + fadeAmount;  //Takes the value of brightness and adds the fade amount

//The following if statement reverses the brightness when the variable reaches a maximum, 255 or a minimum, 0
   if (brightness <= 0 || brightness >= 255) {
        fadeAmount = -fadeAmount;  //Changes brightness value to its opposite integer value
   }
   delay(30);  //Sets the wait time (in milliseconds) before running the loop
}