Events are at the heart of most pages utilizing JavaScript. For our JavaScript to perform an action, we must wait for a specific event to happen, and then execute the predefined action.
While it was hinted at earlier, events are things that happen on the page. The majority of events utilized in JavaScript applications are user-driven, like: clicking a button, hovering over an element, and pressing a key. However, there are many other events that the browser fires, like: loading all of the resources on a page or going offline. MDN has a good reference of available DOM events.