SOA stands for Start of Authority, and it is a type of DNS record that contains information about a DNS zone, such as the name of the primary name server, the email address of the zone administrator, the serial number of the zone, and other parameters. The serial number is used to indicate when a zone has been updated, and it is incremented by the primary name server whenever a change is made to the zone data. The secondary name servers use the serial number to determine if they need to request a zone transfer from the primary name server to synchronize their data.
References: [CompTIA Cloud+ Study Guide], page 207.