SOA record metrics:
Refresh: This is the number of seconds between update requests from secondary and slave name servers. If notifies are enabled this is a fairly redundant feature.
Retry: This is the number of seconds the secondary or slave will wait before retrying when the last attempt has failed.
Expire: This is the number of seconds a master or slave will wait before considering the data stale if it cannot reach the primary name server.
Minimum: Previously used to determine the minimum TTL, this is used for negative caching. This is the default TTL if the domain does not specify a TTL.
DNS Client TTL (time to live) - The number of seconds any DNS record is cached on a DNS server that queries your NS before expiration and return to your (authoritative) nameservers for updated information. This is also in theory the number of seconds a local DNS Client will keep queries cached before re-performing the query although it is largely not followed by browsers.