All websites created in the Weebly Website Builder must be published to your web hosting.
When a user publishes a site, Weebly will FTP some static files (index.html), proxy files (a few PHP files) and .htaccess to the location you've specified for that site.
The php file is very small and acts as a proxy. The first request to the published site is responded to by the php file (via the 404 handler specified in .htaccess). The php file makes a call to Weebly for the contents of the page. The contents are then returned to the user's browser.
At the same time, the php file caches the content for each subsequent request to avoid future trips to Weebly for the contents. Upon next publish of the site the cache is cleared and the process starts again.
Setting FTP Credentials
In most cases the FTP Publishing Credentials is set automatically when you make a purchase together with hosting plan.
Should this fail for any reason or need to be modified, please submit a ticket to support.