Every zone has exactly one SOA record. It names the primary nameserver, gives the mailbox of the person responsible (written with a dot instead of the @ sign), and carries a serial number that increases whenever the zone changes. Secondary nameservers compare serial numbers to decide whether they need to fetch a new copy of the zone.
The refresh, retry and expire timers control that replication between primary and secondary servers. The last field matters to everyone else: it sets how long resolvers cache a negative answer, that is, the fact that a name or record type does not exist.
That negative-caching TTL explains a common surprise: if someone looks up a hostname just before you create it, their resolver may keep answering "does not exist" for that many seconds even though the record is now in place.
Example
example.com. 3600 IN SOA ns1.dns-host.example. hostmaster.example.com. (
2026092001 ; serial
7200 ; refresh
3600 ; retry
1209600 ; expire
300 ) ; negative-caching TTL