Computers find each other by numerical addresses, but people prefer names. DNS is the system that translates a domain name into the address of the server that should answer. It also says where your email should be delivered, so it quietly underpins both your website and your inbox.
DNS matters to a business because mistakes in it can quietly break things: a website that does not load, or email that never arrives. Changes are not always instant, as records can take a while to spread across the internet. Keep a note of who controls your domain and your DNS settings, because that person holds the keys to your online presence.