WHOIS is a public directory where you can look up "who is" the responsible person(s) for a domain or IP address. The governing body of domains, ICANN, requires valid contact information (also called WHOIS) to be listed for every domain name. In addition to including valid contact information, it is important to know that this information is freely & publicly available and, unless you have domain privacy, anyone can look it up.
However, since the GDPR and the Data Protection Act, 2018 has taken effect in May 2018, most information about who owns the domain has become hidden. This is a temporary decision, while ICANN figures out how best to comply with GDPR in regards to the WHOIS database. This means that ICANN can still decide to make (part) of this information publicly available again in the future.
For more information on GDPR you can read about it here: https://ec.europa.eu/info/law/law-topic/data-protection/reform/rights-citizens_en
WHOIS details contain invaluable information for knowing where a domain is registered, what the nameservers are set to, and other things. WHOIS details can be checked by running the command ‘whois yourdomain.com’ in a terminal session, or by visiting any online WHOIS checker such as whois.com. You can also view and edit the WHOIS details in CHI under the ‘Domains’ section. See How To Change WHOIS Information for more information on this.