What does propagation mean?

When you make changes to settings on a nameserver these changes need some time to be propagated. I will try to explain what exactly happens and how you can see how long propagation will take in the worst possible case.

I don't want to read all this, take me to the TL;DR version!

The whole story

If you wanted to access a website in the early days of the internet you had to type in an IP address. So for example if google had existed back then you would have entered an IP address like http://172.217.21.206 in your browser and google would have come up.

Nowadays there are millions of websites out there and for us humans it is easier to remember names like google.com or amazon.com instead of random IP addresses. Luckily some smart people recognized this in the 1980s and created the DNS system. You can read about the history of DNS in wikipedia if you are interested.

There are two types of nameservers. One type is called authoritative nameserver and stores the information for a specific domain, the other type is called non authoritative nameserver and caches information.

Today your clients (PC, Mac, phone, etc.) nameservers, the non authoritative type, are usually configured through a service called DHCP or if you are in a IPv6 network through something called router advertisements. You can find out which nameservers are configured in the network settings of your operating system. Most operating systems also maintain a local cache of DNS information.
The second link in this chain can be your router or your ISP depending on your network setup.
Usually your ISPs nameservers or the ones configured in your router are the last step of caching in this chain. You can also configure your operating system to use other nameservers and skip your router or ISP. Googles nameserver 8.8.8.8 and 8.8.4.4 are a good choice but there are alternatives like Quad9.

So for example when you want to visit GHOST_URL/ your operating system checks the local cache for a matching IP address, if this fails it will ask the next nameserver to resolve the address. Let's say your operating system is configured to use 8.8.8.8 as a nameserver. So if 8.8.8.8 does not know the answer it will ask the root nameservers, the root nameservers will respond with a referral to the eu nameservers, the eu nameservers know the authoritative nameserver and will answer with a referral to ns1.monikerdns.net, ns2.monikerdns.net, ns3.monikerdns.net and ns4.monikerdns.net. One of these is now chosen by random and it will resond with the following:

dnslog.eu. 28800 IN A 37.120.164.254

That is the A record of the domain dnslog.eu. It points to the IP address 37.120.164.254 and has a TTL of 28800 seconds.

That means that every caching nameserver (so the one on your operating system and 8.8.8.8) in the chain will cache that information for 28800 seconds or 8 hours.

So if you just visited the website and then changed something in the DNS configuration of your domain chances are that you will see the change only after 8 hours or whatever the TTL on the authoritative nameserver for your domain is set to. 8 hours is a pretty common value, but it can also be several days or weeks.
Most registrars or nameserver providers offer you the possibility to reduce the TTL to a value like 5 minutes. Which is very useful if you make changes to the configuration and want to see the results quickly.

TL;DR

There are two types of nameservers. One type is called authoritative nameserver and stores the information for a specific domain, the other type is called non authoritative nameserver and caches information for how ever long the TTL is set on the authoritative nameserver. Usually you can change the TTL. The nameservers can be and are usualy chained in some way.