like I'm 5 (0r 4 or 6).

Okay then.

Before dawn of technology advancements that we have today, people did stuff in a very different manner, for the sake of this explanation, I will call it "primitive"

As brilliant as human beings are, they often forget little things (little because may not have higher priority at that particular time) and dates is one of them.

Even now, if you happen to forget today's date, and do not have means for referring that (like looking at your smartphone or watch, some digital billboards and whatnot),

what you would naturally do is refer back/forward, to the closest (recent/upcoming) date and day where a memorable event occurred/will occur. Events like your cousin's birthday, trump impeachment, the coming football derby or the coming elections date. then you start counting with your fingers towards/backwards to the current day. This is "primitive"

These variations of calendars that currently exist today have their own sort of "memorable event".

The most widely used today is AFTER CHRIST (AD). (Of which, to go back past that, they should have used count backwards tactic, i.e. -1, -2, -3, -4; Eg: -4AD; but instead, -4AD becomes 4BC which is BEFORE CHRIST. That is why counting forwards in BC, number decreases 😏 )

To answer your question;

"Year zero" is the year where that particular memorable event occurred.

But as I demonstrated above, we use that year as a reference to count forward/backwards the following/past years.