Your DNS (Domain Name Servers) number are a set of numbers recorded at your domain name registrar that tells the internet which server to go to to read your website (and other) files. There are DNS “Records” for your website (A-records), your Mail (MX- Records) and host of other records in your DNS settings. If all DNS numbers are pointed to your host, your mail and your website will all be handled at that host. If you are moving from one web host to another, these numbers need to be changed to point to your new server. And then they need “resolve” by propagating across the internet. This can take from a few hours, up to a few days.
What is DNS propagation and why does it take so long?
In order to resolve a domain name, it must be registered and pointed to at least two name servers which will host the DNS zone file for the specific domain. These servers are authoritative so when someone on the Internet “asks” for the IP address of the specific domain, the request will first go to the root name servers for the requested TLD (top level domain, for example: .com, .net, .org, etc.). They will reply with the authoritative name servers for the requested domain and the request will go to the server hosting the file with the DNS records where the IP will be resolved from.
Oh, there’s more… Read the complete article at Siteground including detailed instructions on how to clear the DNS cache to help your site resolve faster.