Whois is a widely used internet record that lists all the people who own a domain and how to contact them.
It is like a huge phone book for the web where you can access the owner of every site. It includes the name and address of the registrant, creation date of the site, the domain servers, and other bits of information about the creation of the site.
Having this information can be very useful. It means that people can contact you even if parts of the site are not working. The ability to find out the information means you are more likely going to be contacted if something is not working or if you are breaking any rules. This can be very helpful due to it allowing you to make corrections and improve your site/business.