The game is a Universal Windows Platform Client for Niantic's Pokemon Go, which the developer says will give you "the ability to play in the same game-world as your friends that are playing with an Android or iOS device." That is true even though the app, called PoGo-UWP, doesn't offer the same AR and 3D visuals found in the original game.
According to the developer, you still need to open a Pokemon Trainer Club account, and you still need to open the iOS/Android game to select your starter Pokemon (you can use Bluestacks to do this). Keep in mind that there are two different methods of installing the game, one for beginners and one for experts. Unless you're a developer, stick with the former.
Another thing to consider is that Niantic can put the kibosh on this whole thing by sending out a Cease and Desist letter or pulling access to the API. And this is a Beta build which means that there are some issues. One major one is a bug that crashes the game when you tap or pan the map after catching a Pokemon or visiting a PokeStop.
Obviously there is some heavy demand from Windows 10 Mobile users for this game. The petition that was put up on Change.org requesting that Niantic build a version of Pokemon Go for the platform, now has just under 100,000 signatures in less than three weeks..