From LQWiki

SOA stand for Start Of Authority and is used in DNS network configuration files for named using Standard Resource Records to mark beginning of zone data. A typical SOA record looks like

@   IN    SOA  somehost.somedomain.sometld. username.somehost.somedomain.sometld. (
               12345              ; serial
               720000             ; refresh every 200 hours
               1800               ; retry after 30 minutes
               7200000            ; expire after 2000 hours   
               900                ; negative cache is 15 minutes
               )