Recursion is a name-resolution technique in which a DNS server queries other DNS servers on behalf of the requesting client to fully resolve the name and then sends an answer back to the client. Attackers can use recursion to deny the DNS Server service.
Basically what this means is that one DNS server pings another DNS server and then sends that information back to the client. This isn't good because that ping can be taken over by an attacker and be used to hack your website.
It definitely is a bad thing and something you definitely do not want happening.