DNSBL stands for DNS Blacklist or Block List and is a list of IP addresses, distributed throughout the Internet, identifying computers or networks accused of spamming. Many web hosts and Internet service providers (ISP) use these blacklists in their anti-spam filters to help keep spam from reaching their users.
DNSBL have received mixed reviews and provide variable results. In some cases, they go a long way in legitimately blocking spam. On the other hand, as I mentioned above, mail servers accused of spamming may be added to the list, even if they actually are not spamming. Furthermore, servers that are attacked and used by spammers to send spam are also blocked without warning, even though the server owners may have been innocent victims.
This is why you should care. There is a chance that your server could be attacked, used to send spam, and added to a DNSBL, so it is a good idea to know what they are and how to check them to see if any of your IP addresses are listed.
When an ISP downloads a DNSBL and blocks the IPs listed, none of the email messages sent from those IPs will reach users of that ISP. Therefore, if your server ends up on a list, you and your users could potentially be cut off from thousands or even millions of email users.
The good news is that most DNSBL are temporary and drop IPs once the spamming stops or the open relay is fixed on the server. If you find out that your mail is being blocked (often apparent in the email header), you can usually get off the blacklist simply by fixing whatever security problem you have with your mail server.