All DNS records have a TTL property, specifying the amount of time other DNS servers and applications are allowed to cache the record.
When a DNS record is stored in the cache of a DNS server, the record’s TTL is continuously reduced as time go by, and when the TTL finally reaches zero the record is removed from the cache.
When deciding on the TTL, you need to consider how often the record will be changed.
Because of caching, changes to a DNS record will not reach the entire network until the original TTL has expired – a good reason for setting a short TTL.
But caching helps reduce network traffic. The longer the TTL, the longer the record will live in other DNS server caches around the world, and so fewer requests to the original DNS server are needed – a good reason for setting a long TTL.