It’s a bit of plastic, and it “just works”

It’s a piece of plastic that you put a nano (or micro) SIM into in order to make it behave as if it were a larger size. It “just works”; used it to put a nano-SIM into the micro-SIM port of a Samsung Galaxy S III; slightly fiddly, but that’s only because the thing is of course pretty small, but “just worked” and continues to work perfectly.