A fuzzer is an automated software testing tool used in cybersecurity to discover vulnerabilities in programs. It works by repeatedly inputting large amounts of random, unexpected, or invalid data - called fuzz - to the target application. By monitoring the program's response, such as crashes, memory leaks, or unexpected behavior, fuzzers help identify potential security flaws that could be exploited by attackers. Fuzzers are valuable in uncovering issues like buffer overflows, input validation errors, and other bugs that traditional testing might miss. They are widely used in both open-source and commercial security testing processes to improve software robustness and security.