#define BUTTONPIN 2
#define LEDPIN 13

// By default, the LED is not lit.
boolean LED_State = false;

void setup() {
  // put your setup code here, to run once:
  pinMode(BUTTONPIN, INPUT_PULLUP); //defaule value: High/true
  pinMode(LEDPIN, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
  // When the level of the BUTTONPIN is High(default value when unpressed)
  // do nothing but waiting
  while(digitalRead(BUTTONPIN))
    ;
  
  // When the button is pressed, the LED_state is changed and expressed.
  LED_State = !LED_State;
  digitalWrite(LEDPIN, LED_State);
  delay(300);
}