Full name of fofa: fofa-Cyberspace Asset Search Engine.
The official website describes it this way:
FOFA is a cyberspace asset search engine launched by WhiteHatHub. It can help users quickly match cyber assets and speed up the follow-up process. For example, vulnerability impact range analysis, application distribution statistics, application popularity ranking statistics and so on.
Simply understood:
Knowing how a product is deployed on the Internet, getting all the sub-domain websites of a root domain, confirming a company based on its IP, finding other websites that are on the same IP as him based on a sub-domain website, scanning the whole network for vulnerabilities, and the scope of influence of a new vulnerability across the network.
A locally enhanced version of shodan (an international version of the Cyberspace Asset Search Engine)