The Domain Name System (DNS) allows the translation of friendly domain names (such as www.webinabox.net.au) into the numeric computer address that is responsible for that domain (such as 184.108.40.206 or 2403:dc00::245).
In some cases, such as MX (Mail eXchange) or SRV (Service) records, the translation result might be another domain name that needs further translation into the final address or a referral to another server that knows the answer.
When a device or program (such as your web browser, or e-mail server) makes a lookup to determine the appropriate server address, these results are cached for a certain amount of time. This cache time is known as the "Time To Live" or TTL. This reduces the demand on the DNS servers, and also helps to speed up the Internet.
Imagine this scenario: you visit our website at www.webinabox.net.au.
This is important to understand if you make a change to the DNS record. If you move the website to another server and make a change to the IP address, any device that has already looked up that domain name within the last hour may re-use the previous result -- they will not see the change until the TTL cache time has expired. Any new clients that had not previously looked up the domain, will immediately see the new record without any delay.
Except that remember back in Step 1, your computer didn't ask the Web In A Box DNS server for the answer directly. It typically asks your Internet Service Provider for the answer, which also remembers answers for their TTL cache time. Thus, even if a specific person or computer had not looked up the domain, they may still see the old record until the TTL cache time expires, if any other user of the same provider had looked up the domain recently.
We generally recommend (and provide a default) TTL time of 3600 seconds (1 hour). This provides a good compromise between performance and the time taken for changes to go through. However if you know in advance that you plan to change a record in the near future, you can modify the TTL to a much lower value (e.g. 60 seconds) so that when you do make the change, it propagates much faster. You can raise the TTL back to 3600 when you make the change.