Nameserver
The Quick Version
The servers that decide who answers DNS questions for your domain.
How it actually works
Nameservers are the authoritative source of your domain's DNS records. Changing nameservers hands DNS control to another provider entirely, instead of editing individual records.
Why does it matter?
If your records look right but nothing works, check whether your nameservers point where you think.
Example
ns1.yourhost.com / ns2.yourhost.com