Your SOA serial number is used as a version number for your DNS zone. For all name servers to be up to date with current version of your zone, they must have the same SOA serial number.
In order for zone transfer to occur to secondary name servers, an ACL must be configured with the correct IP addresses for AXFR secondary name servers. In addition, NOTIFY must be configured to inform the secondary name server of zone changes made on the primary name servers.
When a change is made to the zone on the primary name servers, the SOA serial number is incremented and with NOTIFY configured, the secondary name servers are informed of the change. As long as the SOA serial number on the secondary name severs is lower than the SOA serial number on the primary name servers, a zone transfer will occur. If the SOA serial number on the secondary name servers is higher than the SOA number on the primary name servers, a zone transfer will not occur. The SOA serial number on the primary name servers would then need to be changed to a serial number higher than the SOA serial on the secondary name servers to correct the problem.
If the primary name server does not support NOTIFY, the secondary name server will query for the SOA serial number based on the refresh value configured in the SOA record.