Here's an example of an iCal subscription URL:
The "webcal" URL will work with all Apple products (iCal, iPhone or iPad). When you share your calendar via email, an "http" version of the URL is also included ("webcal" is simply replaced with "http"). The "http" URL will be required to initiate a calendar subscription on some non-Apple devices.
The "private link" or "private URL" is required when subscribing to calendar applications that don't support authentication and/or have a URL-length limitation, most notably Google Calendar.
Here's an example of a private URL:
As you can see, your FileMaker server address is part of the private URL. The long string that follows "SeedCode_Subscribe/" is basically a bunch of random characters that uniquely identifies a calendar record in your SeedCode_Subscribe.fmp12 file. Your SeedCode_Subscribe_Setup.fmp12 file must be hosted on a FileMaker Server so calendars that subscribe via the private URL can gather information about your database and update calendar content. Also, anyone who has this private URL (or guesses it) will be able to subscribe to your calendar, and the private URL uses the FileMaker Username and Password you configured in the calendar's record in your SeedCode_Subscribe_Setup.fmp12 file: thus users are logged in automatically. So be careful whom you send it to, and ask your users not to give it to anyone else.
Learn more about private URLs and working with Google Calendar.