Now, if servers had to keep doing these same lookups over and over again they would become quite slow. So 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 a DNS setting called the "minimum" or "TTL" (time to live) which is found in a domain name's SOA (start of authority) record.

The effect this has on making changes to a domain name's DNS (like changing an IP address) is that the change 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 and will depend on the minimum/TTL setting.

So, if the minimum/TTL is set to 3 hours, then 3 hours is the longest amount of time it will take for the new address to be used by everyone on the internet.

This isn't a problem when you are using a static IP because the address does not change, but when you are using dynamic DNS you don't want people to be waiting for 3 hours before they can get to your website because your IP address just changed.