When visitors type your domain name into their web browsers, their request is referred to a nameserver that indicates the physical location (the IP address) of your domain on the Internet.
When your site location changes to a new IP address, or when you open a new site with a new domain name, nameservers around the world must be notified of the change.
This process of notification is referred to as propagation and can take anything from 6 - 36 hours but official documentation give it as 72 hours maximum.