It means you have the left pane in "Solo Mode", i. e. when you select one of the tabs the others will close automatically. Right click on any of the sections headers and choose from the menu, you you want to change the mode.
Actually, I found this correct answer:
The triangle to the left of the folder icon is grey ("spotted") if there are no subfolders, and it is solid if there are subfolders. You can click on the solid triangle to collapse the subfolders.
here is the link: