I made a pen based on the lesson to try to illustrate and understand better. It's at the bottom of this post.
Is when an event "bubbles" up the DOM. In the example with 3 nested divs with an event listener on all of them, a click on the inner one will trigger the click listener on all of them. Note that
this is the item the listener is on, but
event.target is always the item that was clicked.
In JS this means that the event will keep bubbling up (or capturing). If you call e.stopPropagation() the propagation stops.
Event listener parameters
The addEventListener() function takes an optional options object. Of these,
once are interesting.
Will allow the event to fire once and then remove the event listener.
I think the easiest way for me to understand this is that when this is true, the events bubble backwards. Try playing around with the pen example to try and make sense of it :)