Reliquary is a (3rd party) plugin for Craft by Charlie Development. Its purpose is to provide advanced search controls and searching ability to the site.
Reliquary is setup by development and should not need to be configured.
Search groups are pre-determined buckets of element types that can be searched. The purpose of a search group is to allow for targeted searching to ensure the results returned are only of a specified type.
An element is a single entity, or item, within Craft. Categories, Assets, Users, Tags are all considered elements. Entries are also considered elements but are designated by their entry type. For example a section that contains two different entry types would contain two different types of elements.
Reliquary is powerful in that it can search all data on the site, if allowed. User accounts can be searched specified, which has the potential of revealing user names, user emails etc if configured. Assets, such as documents that are intended to be hidden, could be revealed in a search if assets are configured to be searchable. By default Reliquary does not search Users or Assets as part of a search group, and must be manually specified.
Each search group passes data through a specified results template before results are sent front end. These templates are setup by development.