A representation image is one that is used to represent a larger collection of related or similar images.
Backlight supports two types of representational images:
The Cover Image is used to represent a collection -- either an album, or album set -- and is displayed in the collection's parent set. For example, in your Travel set, you might use a photo of the Eiffel Tower as cover image for your France collection.
The Hero Image is also used to represent a collection, but appears in the head of the collection itself, usually as a sort of visual introduction.
There are various means of setting up and selecting representative images for a collection, some of them now antiquated. As of Backlight 3.2, the preferred way of setting representative images is via Backlight's Publisher.
While Lightroom users likely prefer to publish images via the Lightroom Publisher Plug-in, the Backlight Publisher is still the best way to set representative images, even for Lightroom-managed collections.
Within the Backlight Publisher:
In an album, click an image thumbnail to open its large view. Open the Image menu to find options for setting the image as Cover or Hero for the album, or for any of its parents.
Upload a custom cover or hero image for an album or album set.
This video demonstrates both of these methods: