It's a driver that lets you use your JoyCons/Pro Controller with your PC with a lot more functionality than the default Windows driver (Analog stick support, Gyro Controls, Combining JoyCons, etc...)
The above text answers the common first big question: "What actually is this thing and why should I use it?", and below is the overview settings, features and comments from its developer @mfosse regarding this awesome new release which allows us to finally fully enjoy the proper usage of your Nintendo Switch JoyCon and PRO controllers on your Gaming Desktop PC or Laptop rigs.
DOWNLOAD and MORE INFO:
JoyCon-Driver v0.98 for PC (via) GBATemp