The term means Before the Common Era. It is a term, which is used solely for the purpose of removing Jesus Christ from the date ranges which are basically from his birth/death. In other words, they use the same dates...just take the meaningful part of why we use those dates out.
Maybe someone can explain what “common era” means? Common with what?
I think it is just a way to assuage the sensibilities of other religions and a euphemism for Before the Christian Era.