Time to Live or TTL refers to when the nameserver checks with the authoritative nameserver to get the latest record. I caches that record for however long the TTL specifies before checking again. The lower the number, the sooner records will be updated.
Many nameservers are set to a default time of 86400 seconds (24 hours). With that setting, it takes 24 hours before all of the DNS servers globally are updated, although some Internet servers will update before others. The lower the number of seconds to live updates, the more often the nameserver will access the authoritative records.
While one’s first inclination might be to lower the TTL number so that nameservers are updated almost immediately, having a setting that is too low can cause too great of a strain on the server. If you change your DNS often, you might want to make the setting lower than one day, but if you only change it once in a while, that default setting might be fine. Some system administrators will lower the setting temporarily if they know they are about to make a big change to critical network services.
Image Source: Wikimedia Commons