What does Ante Meridiem and Post Meridiem mean in time?
They are Latin words ; Ante Meridiem (AM) means before midday and Post Meridiem (PM) means after midday.
AM and PM
Ante meridian simply means before noon, while post meridian means after noon. Each of these two periods have a total of 12 hours numbered, and you can find more details here http://en.wikipedia.org/wiki/Ante_meridiem.