What Does "ExecuteInEditMode" Actually Do?

So, this is my first Unity project and also my first experience with Javascript, coming from a novice background in Python. I am currently building the "Lerpz" tutorial project, and I'm wondering what the "ExecuteInEditMode()" script does? Does it mean that the script will run even when the "Play" button hasn't been pressed? Or does it just mean that it will run within the Editor as opposed to a build? Specifically, shouldn't the menu show up even when the game isn't in "Play" mode?

Here's the code: @script ExecuteInEditMode() //Makes the script also execute in edit mode var gSkin : GUISkin; var backdrop : Texture2D; // our backdrop image goes in here private var isLoading = false; // if true, we'll display the "Loading..." message

function OnGUI() { if (gSkin) GUI.skin = gSkin; else Debug.Log("StartMenuGUI: GUI Skin object missing!");

var backgroundStyle : GUIStyle = new GUIStyle();//creates new GUIStyle object backgroundStyle.normal.background = backdrop;//makes the "normal.background" (a style element) to our background image

GUI.Label ( Rect( (Screen.width - (Screen.height 2)) 0.75, 0, Screen.height * 2, Screen.height), "", backgroundStyle); //creates screen that will remain proportional to screen size GUI.Label ( Rect( (Screen.width/2)-197, 50, 400, 100), "Lerpz Escapes", "mainMenuTitle");//creates a label on the screen

if (GUI.Button( Rect( (Screen.width/2)-70, Screen.height - 160, 140, 70), "Play")) { isLoading = true; Application.LoadLevel("TheGame"); // load the game level. }

var isWebPlayer = (Application.platform == RuntimePlatform.OSXWebPlayer || Application.platform == RuntimePlatform.WindowsWebPlayer); if (!isWebPlayer) { if (GUI.Button( Rect( (Screen.width/2)-70, Screen.height - 80, 140, 70), "Quit")) Application.Quit(); }

if (isLoading) GUI.Label ( Rect( (Screen.width/2)-110, (Screen.height / 2) - 60, 400, 70), "Loading...", "mainMenuTitle"); }

Thanks for any help and sorry for possible noob-ness.