A SOA (Start of Authority) record is a mandatory record used to define the information for the authoritative nameserver serving your zone.
The SOA information consists of the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
The SOA record typically resembles the following format:
example.com. IN SOA ns.example.com. hostmaster.example.com. ( 2003080800 ; sn = serial number 172800 ; ref = refresh = 2d 900 ; ret = update retry = 15m 1209600 ; ex = expiry = 2w 3600 ; nx = nxdomain ttl = 1h )