What does the AD in Christian chronology mean?

AD after a date — as in 1066 AD — means Anno Domini (from Latin, meaning "The year of Our Lord) and counts the number of years from the birth of Christ, supposed to be in the year 0. AD. Prior to this Christians calculate years backwards from the year before Christ's birth (1 BC - I year before Christ) and rising as you get further back in time from this date.