Domain propagation is a period of time ISP (Internet service provider) nodes across the world take to update their caches with the new DNS information of your domain.
Due to DNS caches of different levels, after the nameservers change, some of your visitors might still be directed to your old server for some time, whereas others can see the website from the new server shortly after the change.
Why do I have to wait?
The propagation depends on many factors, among which are the physical location, the Internet Service Provider (ISP) and other influential factors.
Each of the ISP nodes checks its own cache to see if it contains the DNS information of the domain. If it is not there, it looks it up and saves it in order to speed up the loading next time and to reduce the traffic.
How to check if DNS propagation has completed for you?
A commonly used tool that helps provide information is WhatsMyDNS https://www.whatsmydns.net/
