What it is
DNS translates domain names into the addresses machines actually use. A domain publishes several record types: A and AAAA for addresses, MX for mail, NS for nameservers, CNAME for aliases, and TXT for arbitrary text.
Why it matters
Address records point at the servers. MX records name the mail provider. TXT records tend to accumulate verification strings from every service the organisation has ever signed up to. Together they sketch out how the domain is run, and who runs parts of it.