The main event loop

You've already seen it. Go back to the previous page and look at the controller code. See the main loop that awaits an event from the user to trigger each action? That's a main event loop. It's the most common pattern for interactive (as opposed to batch) programs. They wait for each event or user action, e.g. a keypress or a mouse movement, and then trigger an action based on it. Most office productivity programs (word processors, spreadsheets, etc.) spend almost all of their time waiting for the user to do something and then just follow a clear set of rules to determine what routine to fire as a result. "The main event loop" is just jargon, not a deep idea, but it's jargon you should know, and even use, e.g. in interviews.