In Adobe Photoshop Lightroom there are many places where you might want to expand or collapse multiple items all at once; the Folders list, Collections list, or keywords list for example.
If you hold the alt-key (option on Mac OS) and click the triangle next to the entry to expand or collapse it, it will expand or collapse that entry and any sub entries that can also be expand or collapsed.
Taking it a step further alt-clicking (option-clicking) on the panels themselves will switch them into solo mode, where only the active panel will be expanded, and everything else will just be a title. You can tell if the panels are in solo mode because their open/close arrows go from being solid to being dotted.
One final tip, control-clicking (command-clicking on a mac) a panel title will expand or collapse all panels on that side of the screen.