A microsite in Sitefinity is a page or group of pages under an existing website that feel like a separate website. You can create a microsite by setting up a page with its own navigation structure under your existing website.
This new top level (or parent) page you've created will serve as the homepage for your microsite. To create additional pages for your microsite, create child pages under this top level page. Please note that you do not need to uncheck "Show in navigation" for these child pages.