A DNSBL is a DNS (domain name service)-based spam blocking list. Some people call them blacklists, while others call them blocklists.
The original (and still primary) use for DNSBLs is to block mail. Most mail servers nowadays have DNSBL support (either built in, or through use of a plug-in) that allows a mail server administrator to block mail from sites listed on a specific DNSBL. The site would choose to do this as part of their attempt to reduce the amount of spam their users would receive.
More recently, DNSBLs are often used as a part of spam scoring system, such as SpamAssassin. If you’re listed on a spam blocklist that is referenced in a spam scoring system, your spam score could be increased by some amount. (The amount varies and is often configurable.) If that, in addition to other scoring tests performed, makes an email’s score rise above a certain level, it could be discarded, or routed to the spam folder.