The Group By tab in the Legend panel shows which categories are inside the feature mapped to the Group By dimension. It is also a useful way to filter your visualization by showing and hiding specific categories and bins.
The top of the Group By list shows you the number of unique categories in the mapped feature and the total number of points that have a value for that feature.
This section shows every category contained in the feature. The list is ordered by the number of points in each category by default. Each category has a toggle button on the left, the name of the category in the middle, and two numbers on the right. The numbers on the right show the number of visible points out of the total number of points in the dataset that are in this category.
The list is interactive and is used to quickly show or hide particular bins. Clicking on the name of the category will show exclusively that category and hide others. Clicking on the circular toggle button will toggle that bin on and off.
Clicking the Reset button at the top right of the panel will show all categories again.