The extension list contains all types (filename extensions) occurring in the directory tree. For every file type the column "Bytes" shows the sum of the file sizes. By default, the extension list is sorted descending by this column. So at the top are those files types, which all in all take up most space on the hard disk.
12 colors are assigned to the 12 file types, which take up most space. The rest is grey. The treemap shows the files in these colors.
Click on an extension: The treemap highlights all files of this type.