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. See an example of the difference between the tools here.
The number next to each Browse item indicates how many photos it contains; in the example below, you can see there are 2017 photos with no star rating. The blue bar indicates the relative proportion of number of photos to the total amount of photos in the Catalog.
Use the toggle-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.