Do a Google search for propagation and you will get the following definition:
When it comes to the internet the same basic concept applies. Any time you make changes to a website or DNS you will have to wait for propagation. In other words, you have to wait for the change to spread into new regions. Just like the spreading of a belief or practice takes time, so do DNS changes. What happens is that, most internet service providers will cache your sites IP address in order to allow it to resolve faster. The reason for DNS propagation time being so long is because each internet service provider has to refresh its cache in order to record the new IP address or DNS record for your website.
Normally propagation doesn’t take longer than 24-48 hours, however in the worst case scenario it can take up to a maximum of 72 hours for propagation to complete. There are DNS propagation checkers that can help you check how far your DNS has propagated. If you want to check for yourself take a look at https://www.whatsmydns.net/ where you can check each individual DNS setting and how far it has propogated across the world.