From the Windows Store listing:

Now you can buy and use mobile data at your own convenience. This is the easiest way to get online using a trusted nationwide network. Purchase a plan via Windows Store anywhere network coverage is available. The Cellular data app can get you connected at any time with a secure, high-speed mobile connection.

This app is designed to work solely with specific Windows 10 devices and requires a Microsoft SIM card. Service is available in some markets and offers are for domestic plans only – international roaming offers will be available soon.