A Folder Link never changes, and will always serve the Flipbook that was most recently uploaded to it.
This is a handy feature if you publish a series of Flipbooks often, and want to ensure that your readers always have access to your latest content (i.e. a weekly magazine or daily newspaper).
When using the Folder Link, the Flipbook's "published date" is what determines what the most recent Flipbook is.
Pro Tip: Let's say you have a Flipbook that you want to show up regardless of when you upload any other Flipbooks to a Folder. To do this, you would have to edit the published date of that Flipbook (and the other, more recent Flipbooks uploaded to the Folder) so that it would be the most recent.
A Folder link is of the format "subdomain.yourdomain.com/t/XXXXXX", where XXXXXX is the Folder number. You will know that it is a Folder link because of the t in the URL - Flipbook links (a URL that will always load up a specific Flipbook) will have an i instead of a t in the URL.
You can access the Folder link from the Manage Flipbooks screen as show in the below screenshot: