DNSBL stands for Domain Name System Blacklist, also known as a Real-time Blackhole List (RBL). It is a database of IP addresses that have been identified as sources of spam or malicious activity. DNSBLs are used by email servers and other internet services to block incoming traffic from these blacklisted IP addresses, thus reducing the amount of unwanted and potentially harmful messages or traffic. These lists are constantly updated and maintained by various organizations and are an important tool in the fight against online threats.