The meaning of BC and AD refers to The birth and life of Jesus Christ. This is a way to measure time based on the life of Jesus. BC stands for 'before Christ' and refers to the time before the birth of Jesus Christ.
AD stands for 'anno Domini' and translates to 'in the year of our Lord' this time measures everything after Christ was born. Although these are used in the Bible to measure time they are not always necessary to add to years.