IIS is a set of Internet-based services on Microsoft servers. It works on the server to enable web server services, like FTP, SMTP, HTTP/HTTPS. You can set it up for a company website, to provide an FTP site for transferring files between remote users, and much more. If you are tempted to check it out, this is a pretty good site for a wide range of information:
It stands for Internet Information Server.
It is a webserver like Apache or Tomcat.
You use it to host website.