For information on a particular field like this one, the best way is probably to press F1 for Contextual Help on the window in question. F1 is available for all windows. For example, this help topic for this display is a bit lengthy but towards the end it reads:
If Remove duplicates was selected in the project properties, the # column in the search results contains the number of entries that are identical in a given file.
Also in the Help menu there are links to the online help and to the User's Guide.
But for a quick yet comprehensive overview of Xbench features, we strongly recommend watching the Getting Started playlist.
Also, besides the videos in the playlist, there are also many useful 5-minute videos in our YouTube Channel.