Nameservers used by a home computer to look up a domain name do not simply find the information and then discard it after returning it to the home computer. If they had to keep doing the same lookups over and over again, the speed of email and web connections would become quite slow. Instead, they store the answer for a while so it can be used again the next time someone enters the same domain name in a web browser or sends an email.

This process of storing the answer is called "caching," and the length of time the information is cached depends on the DNS setting called the "minimum" or "TTL" which is found in the SOA section of the zone file. The effect this has on making changes to a domain name's DNS is that the new information will not be used until the old address information is removed from the cache. The process of waiting for old information to be removed from the cache so that the new information is seen by everyone on the internet is called "propagation".

If the speed of propagation is an important issue in a given change, 3 hours (or the current minimum SOA setting if you've not left it at the default) before you are going to make the change, you will want to drop the minimum value to something lower, say 20 minutes or an hour.

Wait the current minimum to allow that change to fully propagate, and then make the change to the DNS you are concerned with and raise the minimum back to 3 hours. This will allow the change to the DNS to propagate based on the lower setting, and then return settings to normal for optimal resolution speed.