I've never heard of this before, but here's what I found on the web:
At the top of the File List pane, click View | Toggle Overlay Mode to control the visibility and color. Toggle through the available modes with this ] key.
Alternatively, click View | Highlight Overlay to select which icons appear in color. Toggle through the modes with this [ key.
For more control over overlay icons, such as whether individual icons are displayed, go to Tools | Options. In the Options window, under File List, choose Thumbnail Info.