It's an expansion card, like this.
Basically adds a WiFi card to your motherboard, allowing it to connect to a WiFi network.
This is a good option, so long as you don't need WiFi 6. Also, keep in mind that the included Bluetooth will need an open internal USB2 connection to work, which the 790 does not have, unless you wanted to get creative with the front panel connections, can find an adapter to connect it outside the case to one of the rear ports, or you've re-cased the system and you're using the front panel adapter that breaks everything out into standard connections.
If you don't care about Bluetooth, then you don't need to do anything and the WiFi will work without the USB2 connection.