They don't really 'do' anything. They are, as the name suggests, simply something to fidget with. (Think of popping bubble wrap, for relaxation or simply for the fun of it, and you're thinking along the right lines).
They started off with fidget spinners four years ago but now there are various different types (including some which are remarkably similar to popping bubble wrap except that, unlike bubble wrap, you can repeat the popping time and time again).