The full form of SOA is Start of authority. The SOA record indicates that this DNS name server is the best source of information for the data within this DNS domain. It contains the information about the primary nameserver of your domain and also contains the email address of the person responsible for the zone also contains following properties:

 
1) Serial number: - This number is automatically increased by Simple DNS Plus when changes are made to the zone or its records

2) Refresh Interval: - How often secondary DNS servers should check if changes are made to the zone. This value determines how often secondary/slave nameservers check with the master for updates. 

3) Retry Interval: - The retry value is the amount of time your secondary/slave nameservers will wait to contact the master nameserver again if the last attempt failed.

4) Expire Interval: - This is how long a secondary/slave nameserver will wait before considering its DNS data stale if it can't reach the primary nameserver.

5) Minimum (default) TTL: - This value used to determine the default (technically, minimum) TTL (time-to-live) for DNS entries, but now is used for negative caching.