An SOA record is a Start of Authority. Every domain must have a Start of Authority record at the cutover point where the domain is delegated from its parent domain. SOA record is automatically added for every domain.
Every domain name has an SOA record in its database that indicates basic properties of the domain and the zone that the domain is in. The SOA record contains:
The host name for the primary name server for the zone.
An e-mail address of the person who is responsible for the domain.
The serial number for the zone.
The refresh interval. This is how often, in seconds, the secondary name servers check with the primary name server to see if any changes have been made to the domain's zone file.
The time, in seconds, a secondary server waits before retrying a failed zone transfer. This time is typically less than the refresh interval.
The time, in seconds, before a secondary server stops responding to queries, or "expires" a zone, after a lapsed refresh interval where the zone was not refreshed or updated.
The minimum time-to-live (TTL). This value is supplied in query responses by servers for the zone to inform others how long they should cache a resource record provided in an answer.