When you want to visit a website you type a website address in the address bar of your web browser. At this point your web browser makes a inquiry to Domain Name System (or DNS). This is how https://bluestar.london becomes a request to a server on the Internet with 18.104.22.168 IP address.
DNS is needed for any website to operate propertly – without DNS visitors would need to memorise cryptic number sequences.
As a website owner you need to ensure your DNS is configured and operational at all times – if it is misconfigured or if it is not working properly – people won’t be able to visit your website.
In addition DNS is used for e-mail delivery and some other services.