PM means post meridiem. That's Latin, but you don't have to remember it. If you simply remember the P means 'post', and the 'M' means the exact middle of the day (12 noon) then you know 12 PM is 12.00 (after noon hence afternoon) and 12 AM is after midnight or 0.00 (the hour after previous day's 23.00 / 11 PM). Then you just need to remember morning is AM, afternoon is PM, evening is PM, and night is AM. That's it. Yeah, I find 24H system easier, but its what my native locale uses, so I am biased... (as is everyone else)
This is in fact so simple to remember that street signs in San Francisco, where AM/PM is native, use 11:59am/pm or 12:01 and never 12:00 to avoid ambiguity. Street cleaning from 12:01am to 3:00am, for example.
Only thing to remember there is: .00 means new hour. Which the number before .00 denotes.
Caters to lowest common denominator.
I had all this explained on elementary school (which was boring and slow as fuck) during English class, and we don't even use 12H system in The Netherlands. It is quite frankly as simple to remember as a logic (tho not necessarily common) grammar rule.