Figure 1: Sort Order icon
If you click on it, it will reverse the order of the window. The exact order depends on how the window is arranged. Whichever column heading is selected is the criterion by which the window will be sorted. For example, if you have the window sorted by date, then clicking the Sort Oder icon will reverse the date-ordering of the file names.
This Sort Order icon only appears in List View under versions Mac OS 8.1 or later. Balloon Help contains information about this and many other Mac OS features. This feature can also be toggled with AppleScript.
Rearranging The Order Of Columns
Starting with the release of Mac OS 8.5, it is also possible to rearrange the order of the columns in a Finder window. For more information on this topic, please see the following article:
Article 24659: "Mac OS 8.5: Columns In Finder Windows Rearranged"