When someone goes to your site, the first page they see will be the one with the file name "index.html".
If someone includes a specific file name when they type a web address, such as www.site.com/special.html, then they'll be given that specific page. If they just type an address without including a file name at the end, for example www.site.com, or www.site.com/foldername, then they'll be given the index.html file that sits at that level. (If they typed www.site.com/index.html they'd also get the index.html file of course, but they don't need to type all that.)
This is standard behaviour for web servers - although some (not many these days though) will also consider files called "welcome.html" or "home.html" to be valid alternatives if there is no index.html file available.
Always make sure that the page you want served by default from any folder, including the top level of your site, has the file name set to "index.html". Look in the Inspector palette when editing that page to check.
Note that the file name is different from the page's title, which is what is shown in the browser's window titlebar.
Freeway 5 automatically sets the filename for first page in your site and folders to index.html, although you can always change this using the File field in the Inspector palette at any time.
