A.D. are the initials of the Latin words "Anno Domini" which mean, "Year of our Lord" just as B.C. means "Before Christ."
To be politically correct these days, the usage has changed to "C.E." meaning "Common Era" referring to the year 0 forward to our present time, and "B.C.E." meaning "Before the Common Era" referring to the time before the year 0.
I hope you have found this helpful.