Need help with code.

This code adapt from @Robin2 "Demonstration code for several things at the same time"
http://forum.arduino.cc/index.php?topic=223286.0

// This code adapt from Robin2 "Demonstration code for several things at the same time"
// http://forum.arduino.cc/index.php?topic=223286.0
// this sketch does the following

//    it turns the Led (buttonLed connected to pin 11,12,13) on or off whenever a button
//       connected to pin 2,3,4 is pressed


//  One leg of each LED should be connected to the relevant pin and the other leg should be connected to a
//   resistor of 470 ohms or more and the other end of the resistor to the Arduino GND. 
//   If the LED doesn't light its probably connected the wrong way round.


//    The use of millis() to manage the timing of activities
//    The definition of all numbers used by the program at the top of the sketch where 
//       they can easily be found if they need to be changed


//========================================

// --------CONSTANTS (won't change)---------------

const int buttonLed1_Pin = 13;      // the pin numbers for the LEDs
const int buttonLed2_Pin = 12;
const int buttonLed3_Pin = 11;

const int buttonPin1 = 2; // the pin number for the button
const int buttonPin2 = 3; // the pin number for the button
const int buttonPin3 = 4; // the pin number for the button

const int buttonInterval = 300; // number of millisecs between button readings

//------------ VARIABLES (will change)---------------------

byte buttonLed1_State = LOW;             // used to record whether the LEDs are on or off
byte buttonLed2_State = LOW;           //   LOW = off
byte buttonLed3_State = LOW;

unsigned long currentMillis = 0;    // stores the value of millis() in each iteration of loop()

unsigned long previousButtonMillis = 0; // time when button press last checked

//========================================

void setup() {

  Serial.begin(9600);
  Serial.println("Starting LED Button");  // so we know what sketch is running
  
      // set the Led pins as output:
  pinMode(buttonLed1_Pin, OUTPUT);
  pinMode(buttonLed2_Pin, OUTPUT);
  pinMode(buttonLed3_Pin, OUTPUT);
  
      // set the button pin as input with a pullup resistor to ensure it defaults to HIGH
  pinMode(buttonPin1, INPUT_PULLUP);
  pinMode(buttonPin2, INPUT_PULLUP);
  pinMode(buttonPin3, INPUT_PULLUP);
   
}

//========================================

void loop() {

      // Notice that none of the action happens in loop() apart from reading millis()
      //   it just calls the functions that have the action code

  currentMillis = millis();   // capture the latest value of millis()
                              //   this is equivalent to noting the time from a clock
                              //   use the same time for all LED flashes to keep them synchronized
  
  readButton();               // call the functions that do the work
  switchLeds();

}

//========================================

void switchLeds() {
      // this is the code that actually switches the LEDs on and off

  digitalWrite(buttonLed1_Pin, buttonLed1_State);
  digitalWrite(buttonLed2_Pin, buttonLed2_State);
  digitalWrite(buttonLed3_Pin, buttonLed3_State);
}

//========================================

void readButton1() {

      // this only reads the button state after the button interval has elapsed
      //  this avoids multiple flashes if the button bounces
      // every time the button is pressed it changes buttonLed_State causing the Led to go on or off
      // Notice that there is no need to synchronize this use of millis() with the flashing Leds
  
  if (millis() - previousButtonMillis >= buttonInterval) {

    if (digitalRead(buttonPin1) == LOW) {
      buttonLed1_State = ! buttonLed1_State; // this changes it to LOW if it was HIGH 
                                           //   and to HIGH if it was LOW
    }
    if (digitalRead(buttonPin2) == LOW) {
      buttonLed2_State = ! buttonLed2_State; // this changes it to LOW if it was HIGH 
                                           //   and to HIGH if it was LOW
    }
    if (digitalRead(buttonPin3) == LOW) {
      buttonLed3_State = ! buttonLed3_State; // this changes it to LOW if it was HIGH 
                                           //   and to HIGH if it was LOW
    }
    previousButtonMillis = millis();
  }

}

//========================================END