Offering Collections allow you to take a collection of Offerings and make them available in a single widget (or on a single, standalone page), wherein visitors are presented with a menu of Offerings to choose from, rather than a single one.
To create an Offering Collection, first you'll need to create the Offerings you want to be part of it. Then visit the Collections tab and click +Collection:

Setting up a collection is a snap. Just give it a name, pick which of
your Offerings should be included, sort them however you like, and
that’s it.

Once your collection is created, you’ll find the same ways of embedding that Offerings have: a <script> or <iframe> tag for a widget, as well as a URL for linking to a fully hosted page.
Like with Offerings, Offering Collections are branded as your own in both widget and full-screen mode.
Offering Descriptions
With Offerings lined up in a sort of “menu” format, it’s nice to be able to show your would-be clients (who will be perusing said menu) a little more than just the name of each one. This is where the “Descriptions” part of an Offering is useful:

Whatever blurbs you set for your Offerings appear in the “Choose” step for the Offerings in a Collection. If you set a longer description (using the WYSIWYG editor just below), that will be accessible as well from that menu.