Each file group consists of space that is used and space that is free (not allocated to any tables/indexes).
The chart values can be displayed in megabytes or as a percentage of disk space. Use the File Groups Space list to change the values displayed on the chart.
The file groups displayed can be restricted by clicking the database(s) of interest in the Databases grid at the top of the drilldown. They can then be further restricted by clicking on just the File Groups of interest in the File Groups grid in the middle of the drilldown. Hold down the Shift key to select a block of multiple records, or the CTRL key to select individual records.