DNS Domain Name System
The internet's phonebook — translates thern.io into an IP address like 185.97.7.85.
When you type a URL in the browser, your computer first asks a DNS server: "what's the IP for thern.io?". The answer is cached so you don't have to ask every time.
Common record types: A (IPv4 address), AAAA (IPv6), CNAME (alias), MX (mail servers), TXT (notes — used for verification, SPF, DKIM).