Browse view provides a way to navigate through the metadata in your catalog. This is the opposite of a filter. Filters start with all photos and narrow it down. Browsing starts with no photos and expands what shows based on which metadata you choose to browse.  

The number next to each item indicates how many photos it contains (2017 photos with no star rating, for example) and the blue bar indicates the relative proportion of items. 

Use the triangles to collapse or expand your view of each item in the browse tree, and click to select an item or the item’s category. If you want to browse more than one item, extend your selection by holding down Command/Control (macOS/Windows) and then clicking on the next item. 

Tip: holding down the Shift key while expanding part of the browse tree will expand that item and all of its branches. (This also works for tree views in the Filter system.)