rblcheck is a very basic interface to RBL-style DNS listings such as those operated by the MAPS and ORBS projects.
The general idea behind RBL-style listings is rapid lookup of IP addresses using DNS (for example, for blacklisting IP addresses because of abuse).
Each IP address is reversed and has a domain name attached to it; for example, the IP address 127.0.0.2 would become 126.96.36.199, and then a domain such as "relays.visi.com" would be added to it.
You would then try to resolve the result (ie. 188.8.131.52.relays.visi.com); if you receive a positive reply, then you know that the address is listed. Further information can also be queried, such as text descriptions of why the address was listed.