A modifier key is a key on a computer’s keyboard that is only used in conjunction with another key. Modifier keys are often used with other keys to perform computer keyboard shortcuts and other commands. For example, in many text editor programs pressing the Ctrl+A will select all text.

Command	Key in Windows	Key in OSX
KEY_LEFT_CTRL	Left CTRL
KEY_LEFT_SHIFT	Left SHIFT	Left SHIFT
KEY_LEFT_ALT	Left ALT	Left OPTION
KEY_LEFT_GUI	Left WIN KEY	Left COMMAND
KEY_RIGHT_CTRL	Right CTRL
KEY_RIGHT_SHIFT	Right SHIFT	Right SHIFT
KEY_RIGHT_ALT	Right ALT	OPTION
KEY_RIGHT_GUI	Right WIN KEY	Right COMMAND
KEY_UP_ARROW	UP ARROW	UP ARROW
KEY_DOWN_ARROW	DOWN ARROW	DOWN ARROW
KEY_LEFT_ARROW	LEFT ARROW	LEFT ARROW
KEY_RIGHT_ARROW	RIGHT ARROW	RIGHT ARROW
KEY_BACKSPACE	BACKSPACE	DELETE
KEY_TAB	TAB	TAB
KEY_RETURN	ENTER	RETURN
KEY_ESC	ESC	ESC
KEY_INSERT	INSERT
KEY_DELETE	DELETE
KEY_PAGE_UP	PAGE_UP
KEY_PAGE_DOWN	PAGE_DOWN
KEY_HOME	HOME
KEY_END	END
KEY_CAPS_LOCK	CAPSLOCK	CAPSLOCK
KEY_F1	F1	F1
KEY_F2	F2	F2
KEY_F3	F3	F3
KEY_F4	F4	F4
KEY_F5	F5	F5
KEY_F6	F6	F6
KEY_F7	F7	F7
KEY_F8	F8	F8
KEY_F9	F9	F9
KEY_F10	F10	F10
KEY_F11	F11	F11
KEY_F12	F12	F12
KEY_F13	SHIFT + F1	FN + F1
KEY_F14	SHIFT + F2	FN + F2
KEY_F15	SHIFT + F3	FN + F3
KEY_F16	SHIFT + F4	FN + F4
KEY_F17	SHIFT + F5	FN + F5
KEY_F18	SHIFT + F6	FN + F6
KEY_F19	SHIFT + F7	FN + F7
KEY_F20	SHIFT + F8	FN + F8
KEY_F21	SHIFT + F9	FN + F9
KEY_F22	SHIFT + F10	FN + F10
KEY_F23	SHIFT + F11	FN + F11
KEY_F24	SHIFT + F12	FN + F12

Example Codes

Program to open a new window. Taken from arduino.cc

#include 

// use this option for OSX:
char ctrlKey = KEY_LEFT_GUI;
// use this option for Windows and Linux:
//  char ctrlKey = KEY_LEFT_CTRL;

void setup() {
  // make pin 2 an input and turn on the
  // pullup resistor so it goes high unless
  // connected to ground:
  pinMode(2, INPUT_PULLUP);
  // initialize control over the keyboard:
  Keyboard.begin();
}

void loop() {
  while (digitalRead(2) == HIGH) {
    // do nothing until pin 2 goes low
    delay(500);
  }
  delay(1000);
  // new document:
  Keyboard.press(ctrlKey);
  Keyboard.press('n');
  delay(100);
  Keyboard.releaseAll();
  // wait for new window to open:
  delay(1000);
}