It is very likely that you have encountered the term "DNS propagation" before but you could not have helped but wonder what it is? You should not worry about it, as you are probably not alone. DNS propagation is a very delicate subject and in order to fully grasp what it means, you need to know how the internet works. Computers are not like humans. They do not understand names, such as Michael, George, William, etc.  Computers understand numbers and this is where the domain name system (DNS) comes in place. 

When you type "amazon.com" in your browser, the computer sees "176.32.98.166". The main purpose of the domain name system is to converts domain names into IP addresses and "tell" the browser which server to query so that you can view the requested site.  You can imagine the IP address, like a street address - each one represents a website, in the same manner, that the street address represents a residential building. 

Let's dive into deeper details of how your browser knows that amazon.com is associated with the IP 176.32.98.166. Each website is located on a webserver that resembles an ordinary computer and is specifically designed to run websites. This computer is placed within a datacenter somewhere in the world and will have a series of IP addresses (depending on the configuration) attached to it.  If this was not complexed enough, we have another layer to this whole equation - DNS servers.  The DNS server's entire purpose is to convert domains into IP addresses.  To do perform this function in the most optimal way, the DNS server stores records in its cache so the next time a person types in "amazon.com" in his browser, it can immediately send him to the server that has the IP address "176.32.98.166".

So, you may be wondering now how DNS propagation fits inside all of this. Well, it is quite simple. Say you are being hosted on server X and your IP address is 123.123.123.123. You are not happy with server X's performance so you decide to move to server Y. Naturally, it will have a different IP address, for example, 213.213.213.213. When you change your domain's nameservers or A record that is currently associated with 123.123.123.123 to 213.213.213.213 and you immediately type in your website in the browser, this will query the DNS server for the IP address. If the DNS server has not yet updated its records or flushed its cache (which happens on a set period of time usually between 24-72 hours), you will be sent to 123.123.123.123 instead. The period which the DNS server takes to update its records of flush its cache is widely known as the "DNS propagation period"

Of course, there are other factors into this whole scenario such as Internet Service Provider(ISP) cache, web browser cache, website cache (if such exists) that add even more layers to the entire process but the most important thing you need to understand is that you have to wait this period out without worrying that something bad has happened to your domain. As soon as the propagation period is over, your website will be fully accessible on the new server.