A record
The Quick Version
A DNS record pointing a domain straight at a numeric server address.
How it actually works
An A record maps a hostname to an IPv4 address, e.g. 76.76.21.21. AAAA records do the same for IPv6.
Why does it matter?
Root domains (no www) usually need an A record because they cannot be a CNAME.
Example
A @ 76.76.21.21