This value configures how often a name server should check it's primary server to see if there has been any updates to the zone which it does by comparing Serial numbers. We issue a warning if it is less than 20 minutes or greater than 12 hours.
Receiving this warning does not mean that your DNS is failing, but may indicate the set SOA Refresh Value is causing your name servers to not be performing as optimally as recommended. This test will warn you if your name server is checking more or less frequently than recommended, as you may be using a high volume of bandwidth or not checking for updates frequently enough.
Refresh: How often a secondary will poll the primary server to
see if the serial number for the zone has increased (so it knows
to request a new copy of the data for the zone). Set this to
how long your secondaries can comfortably contain out-of-date
data. You can keep it short (20 mins to 2 hours) if you
aren't worried about a small increase in bandwidth used, or
longer (2-12 hours) if your Internet connection is slow or is
started on demand. Recent BIND versions (4.9.3) have optional
code to automatically notify secondaries that data has
changed, allowing you to set this TTL to a long value (one
day, or more). [RFC 1912]