SOA record
Start of authority. One per zone. Serial, timers, and who is supposed to be in charge.
SOA is the zone header: primary nameserver, responsible mailbox (a dot in place of @), serial, refresh, retry, expire, and negative cache TTL. Secondaries use the serial to decide whether to transfer.
A serial that never moves means slaves stay stale. A serial that went backwards means they may refuse the new zone. YYYYMMDDnn is a convention, not a protocol rule.