A Word About Time
For many, many years (as far back as the Middle Ages) when historians and other scholars wrote of specific periods in history, they would use the terms AD or BC. So, they might write about an important event that happened in the year AD 927, or another that occurred in 325 BC.
AD is the abbreviation - or shortened version - of the Medieval Latin term that means Anno Domini, or, "In the Year of Our Lord." It is based on the belief that the [assumed] date that Jesus of Nazareth was born was the starting point for this numbering system.
BC is an abbreviation of the English term for “Before Christ.” Any year with BC next to it meant that this event took place before the year of Jesus’ birth.
As you can see, this system of ordering the calendar is based on Christianity and some scholars began to feel uncomfortable with that. There are many people who have a strong religious faith other than Christianity. By using AD and BC it seemed to be accepted that Christianity was the most important religion and other religions were less important.
These scholars began to use BCE (Before the Common Era) instead of BC and CE (Common Era) instead of AD so that the dates would not be tied to the Christian religion. While some scholars still use AD and BC, other scholars are more comfortable with using BCE and CE and you will see all these terms (and more) in the history books you read.