The Serial Number is the third field of an SOA record. Its purpose is to indicate which copy of the zone file is the most current, the primary name servers or the secondary name servers. If the Secondary nameserver's serial number is more than the Primary nameserver's serial number, a transfer of the zone file from the primary authoritative nameserver to the secondary authoritative nameserver will take place.
See also:
SOA Record
Primary Nameserver
Secondary Nameserver