Could you please clarify the question? Do you mean what event the transition is associated with or why it was used as a marker between eras? If the former, B.C denotes Before Christ, whereas AD is Anno Domini (in the year of our lord), which mark in the Julian and Gregorian calendars the year Jesus Christ was born (roughly- I don’t know the exact historical consensus but I believe it’s that Christ was actually born around 5 BC). So for the predominantly Christian western world, the event was and is considered important enough to record as a new era in history. Recently the use of BCE (before common era) is a little more commonly used, but just given the prevalence of records and familiarity with BC and AD the same year ‘0’ is used.