It is covered somewhere...
But I can't find it offhand...
So here's a precis...
The Sets work in a similar way for Plugins and Extensions.
I'll explain it for Extensions...
The use of 'Sets' is to allow you to set up and use collections [aka "sets"] of Extensions and easily witch between them.
So if you do several different kinds of SketchUp modeling, rendering etc, then you can make various Sets of Extensions to suit you needs for each task, and swap between then in a few clicks...
Click on the 'Sets' [blue] button and another 'Sets' dialog opens.
This has several options...
The top one is 'Add' - type in a new Sets name and Add it to the list.
All current Extensions' statuses are remembered.
If there are Sets available the second list shows them, select an item and 'Apply' becomes available.
Click that and the statuses of all Sets are changed to match that named-set.
You will need to restart SketchUp for any Disabled Extensions to be 'unloaded'.
The Revert button swaps back to the last used Set.
The Update button updates the selected named-set to match the current Extensions' statuses.
The Delete button removes the selected named-set from the system.
The Export button lets you save the 'set' file externally so you can then Import it on another Computer using that button.
The Export All button exports a folder of ALL sets which can be Imported using the Import All button on another Computer.
The Import/Import All button uses the Exported file[s], allowing you to sync your Sets between Computers...
If an Extension is included in a Set, but it is not installed on a Computer, then it is ignored.
The Plugins Sets are similar but allow you to choose Plugins which might not have been made as Extensions...