I bet they are referring to recursive DNS queries.
Basically what happens is an attacker uses your server to request dns lookups against a target server. If your DNS server is open (meaning you can lookup any domain with it and not just domains you own/host) then it is possible for it to be used in an attack.
If your running bind it's easy enough to fix:
Edit your /etc/named.conf and toss this in the config and restart it:
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Unfortunately i can't do it, as my accout is suspended for a now.
BTW, i even don't setup and nameserver in this VPS account and usinng namecheap's and pointq.com's DNS hosting.
And if this is important i have setuped ConfigServer Security & Firewall - csf v6.40 in my account.
That is most likely the issue. DNS (Bind) is still active on there but running with a blank / default config which enables recursion.
You'll either want to disable it completely as the other poster mentioned, or restrict it just to "localnets" which is what I recommend as that still allows local programs to perform lookups coming from any IP configured on the VPS.