Nessus, developed by an organisation called Tenable, Inc, is an open-source vulnerability scanner. It is a tool that checks computers to find vulnerabilities that could be exploited. It can be used to check for vulnerabilities that allow unauthorised control or access to sensitive data, misconfigurations, as well as denial of service vulnerabilities and more.
Nessus uses the Common Vulnerabilities and Exposures architecture for easy cross-linking between compliant security tools. It works by testing each port on a computer, determining what service it is running, and then testing this service to make sure there are no vulnerabilities in it that could be used by a hacker to carry out a malicious attack.