DNS is an acronym for Domain Name System
DNS is a server type system that holds the IP address for each website. Note that every ISP (Internet Service Provider) has their own DNS. Think of it as their own personal internet database.
DNS can be managed and created locally through online softwares like Cloudflare.
When you update the nameservers for your domain, it may take up to 24-48 hours for the change to take effect. This period is called DNS propagation.
In other words, it is a period of time ISP reaches across the world to update their servers with the new DNS information of your domain.