The sort order icon resembles a triangle or pyramid. When a Finder window is in list view, this feature toggles the list view between ascending and descending order.
For Example: If the window is sorted by name, clicking the sort order button toggles the contents of the list between alphabetical or reverse alphabetical order. If the window is sorted by size, then clicking the sort order icon toggles the contents of the list between largest-to-smallest, and smallest-to-largest.