Maybe your DNS server is an open resolver that can be used in DNS amplification attacks? That’s my best guess. Like Eric said, we need more details about what your hosting company told you.
Yes, in that case your server is indeed “vulnerable” as in it can be abused to perform distributed amplification DoS attacks using spoofed UDP packets (i.e. the attacker sends specially crafted DNS requests to your server with a spoofed source IP, and your server responds with a much much larger reply to the apparent source, which in truth is the victim).
You can change that by making sure that your /etc/bind/named.conf.options is set properly to allow recursion only from your LAN and localhost. By default, that’s done with this block:
allow-recursion {
localnets;
localhost;
};
(“Recursion” in this context means the DNS server allows and answers requests for zones that it is not authoritative for.)