Every device on the internet has a numerical IP address (like 192.0.2.1 or 2001:db8::1). Humans can't remember those. DNS solves the mismatch: you type modusdom.com, DNS looks up the IP, your browser connects.
DNS is a distributed hierarchical database. When you type a domain, your computer asks a recursive resolver, which asks the root servers, which point to the TLD servers, which point to the authoritative nameservers for that specific domain, which finally return the IP. The whole roundtrip takes 10–50 milliseconds for most domains thanks to caching.
The records that live in DNS are stored as resource records: A and AAAA for IP addresses, MX for mail servers, CNAME for aliases, TXT for arbitrary text (used by SPF, DKIM, DMARC, domain verification), NS for nameserver delegation. When you "set up DNS" for a domain, you're writing these records in the authoritative zone.
