TTL refers to Time To Live for a DNS server. You may find that your server’s default TTL is quite long, perhaps even 48 hours. This means that any changes to DNS zone files will take up to 48 hours to propagate to users. Because that can be very inconvenient when you want to change something fast, you might be tempted to make your TTL value lower. Is this a good idea, and how low can you go?
Ideally, you want to get the job done fast without causing any problems for your server. Making the TTL too long will mean you have to wait anytime you create a new subdomain or perform other minor tasks. Making it too short can add unnecessary load to your server, use more bandwidth and possibly even violate Internet regulations.
To remedy this dilemma, some system administrators will lower the TTL whenever they are doing DNS work and then raise it back to a reasonable level when they are finished. That way, you get your job done in a timely fashion but do not leave your DNS server in a constant state of flux when you are not even making changes.
For more information about this issue, you can read a quite detailed book on the subject online.