Every zone has exactly one SOA record. It names the primary name server, a contact address (with a dot instead of @) and five numbers: serial, refresh, retry, expire and minimum.
Secondary servers compare the serial with their copy and only pull the zone again (zone transfer) when it has increased. The YYYYMMDDnn format is common. The last field, minimum, now defines how long resolvers cache negative answers such as NXDOMAIN.
Classic mistake with hand-maintained zones (BIND): records changed, serial not increased. The primary serves the new version, the secondaries keep answering with the old one.
Example
dig +short SOA example.com