It maintains a number of lists (at present "dsn", "abuse", "postmaster", "bogusmx", "whois" and "ipwhois") which contain domains or IP networks whose administrators choose not to obey the RFCs(pedantically a "Request for Comments"), the building block "rules" of the net.
A listing here simply implies that a site has chosen not to implement the conditions described in a particular RFC. It is, of course, up to other sites to decide for themselves whether or not they wish to communicate with sites that have not chosen to implement, say, RFC2142, and have a working "abuse@domain" address.
Put another way, rfc-ignorant.org does not block anyone. They document who has chosen not to implement certain protocols described in the RFCs, and provide a means for allowing people to determine for themselves if they wish to communicate with non-compliant systems.

There are three ways for IP addresses to get listed.

Operating an open relay or proxy An open relay is any system that will deliver email to any address from any source. Spammers tend to hunt for these systems, and then dump thousands, sometimes hundreds of thousands of emails into these servers, which they then spend hours or days trying to deliver to sites all over the world. In addition to insecurely configured mail servers, web servers with insecure HTML form to mail gateway CGI scripts can be used as "open relays", as can insecurely configured proxy servers. The bottom line is if a server can be used by any non-authenticated source to send email to any address on the Internet, it qualifies as an open relay and may be added to this dnsbl.
Being a dial-up port IP or other dynamic address Rather than hunt for open relays, some spammers will send their email directly from a dial-up account. In general, there's no need for a dial-up user to talk directly to any SMTP server other than that of their own ISP. So njabl.org is compiling a list of dial-up port IP ranges, mostly at larger providers where the abuse seems to be the worst, and they add those to the list. They also include any other IP pools that appear to be dynamically assigned as well as NAT pools, since they are effectively dynamic. Open proxies and trojan proxies are so rampant now that a large percentage of 'open proxies' are actually dynamically assigned dial-up, DSL, or cable modem IPs. These users should all be using their own provider's SMTP servers rather than direct-to-MX as is often done with spam.
Operating a system that directly sends out spam (unsolicited, bulk email, usually of a commercial nature)
The big commercial spammers who swear they got recipient’s name from an associate of an associate who swears you visited their web site and opted into the "all the spam you can handle" service plan.

It publishes via DNS, a list of IP addresses of machines that they know to be either direct SPAM sources or Dial-up (dynamic address) pools which would never be a source of non-SPAM eMail.
DNSRBL has created a series of eMail addresses that they use as 'Honey Pots'. They then use these addresses to post to several bulletin boards and usenet groups. They also have Honey Pot eMail addresses that are used to unsubscribe to SPAM eMail lists. They have never been subscribed to any mail-list of any kind. SPAMMERS scan these bulletin boards and the usenet groups for eMail addresses. Since the only place these addresses have been exposed are these places where SPAMMERS 'harvest' their eMail addresses, when an eMail is received, it is analyzed to verify its source and the eMail server generating the SPAM is placed in our spam list.

It stores IP-addresses of verified open SMTP relays. These relays are, or are likely to be, used as conduits for sending unsolicited bulk email, also known as spam. By accessing this list, system administrators are allowed to choose to accept or deny email exchange with servers at these addresses. Any blocking that occurs, takes place at the receiving system.