Noon is the meridiem that AM and PM reference. At precisely 12:00 it is neither before nor after noon, because it is noon. That makes it hard to intuitively know whether the mid-day 12:00-12:59 should be AM or PM (and vice versa with midnight).

It can obviously be learned, but it's something lots of people have trouble with. I was probably in my teens before I finally got that memorized enough to keep them straight. Lots of things happen at 11:59 PM (due dates being the most prominent example) because it's less potentially ambiguous than 12:00 AM the next day, even though it's functionally the same thing.