Against each folder is shown the total number of files in the folder and all subfolders, the total size of all these files, the total amount of space all the files use and the amount of space all these files would take if the disc had a different cluster size (eg FAT32).

Double clicking the folder line will show the next level down of folders and file types i.e. list all the folders and file types in the folder you have double clicked. Clicking the 'Plus' (+) sign has the same affect.