BC and AD are terms historians use to represent dates in history.
For example, Ramses' II was known for the Battle of Kadesh, a battle that took places against the Hittites in 1274 BC.
BC simply stands for "Before Christ." So in my example above, this battle would have taken place 1,274 years before the birth of Christ.
AD is Latin for "Anno Dominni" and we use it to refer to years after Christ.
So one way to think of 2023, our current year, is 2023 AD.