DNS is short for Domain Name System. It is the service which translates that domain name you type into your browser into an IP address, and tells your browser which server it needs to connect to in order to load the site you want to visit.
When you update the nameservers for a domain, it may take up to 24 hours for all the changes to apply, we call this DNS Propagation. DNS Propagation is basically a period of time ISP nodes across the world take to update their caches with the new DNS information of your domain.
The time it takes for the propagation to happen varies depending on where your server is hosted. It starts by checking your local DNS cache and then expends to other nodes until its fully propagated.
There is no definitive way to tell when the propagation is complete, but there are a few online DNS checkers to track if the information has propagated in different locations across the world.