Multiple Selection
The effect of your selection depends on whether you have the Ctrl or Shift key depressed:
Every time you click an item without Ctrl or Shift your multiple selections are cleared and only the current file is highlighted.
Every time you click a new file it will be highlighted and added to your multiple selection.
If you click a file and then while holding down the shift key click a second file in the panel it will select both files and all files in-between.