What does A.D. mean?
A.D. stands for Anno Domini, which is Latin for "year of our Lord," and it means the number of years since the time of Jesus Christ. That was a little more than 2000 years ago, so the date 500 A.D. means a little more than 1500 years ago.
Some people use C.E. instead. That stands for Common Era, and people use it in order to avoid Christian references. History for Kids feels that since this is a Christian dating system, we don't want to try to hide that. There are other dating systems in the world that non-Christians use, and Common Era makes it sound like everyone should use the Christian system.