Meta has several standard pixel events that you can use to track typical events that happen on websites. But, you can also track custom events.
Custom events are for when standard events aren't good enough. I have several custom events that fire on my website.
Most of my custom events are related to a quality visit. Custom events related to things like playing videos, starting my audio player, scrolling on a page, spending at least a minute on a page, and viewing multiple pages during a session all fire on my website.
Of course, there's a reason for it. I can then use that information to measure the performance of my ads or target people who are executing those events.
Read more about the custom events I use here: