Filters (sometimes called “facets”) are used to refine Content Items in the Content Browser or List Items in the List Browser. Those filters are shown on the right (sidebar) and help refine your search for content; e.g., "Media type" filters let you filter for all content tagged with a media type, and "Department" filters let you filter for content tagged with a department. The filter can use all indexed information saved in the metadata or virtual content. You can only configure filters if you are familiar with the field types and the supported filter Types. Filters are available in Content Browser, List Item Browser, Collections (not editable), Shares (not editable), Users (not editable), and Notifications (not editable). Fotoware Alto offers a configuration editor. 

The technical term for this kind of filter is Aggregation.