Telemetry can be used to communicate with QGroundControl and is very useful especially for tuning for example as parameters can be changed without plugging in a cable each time.
3DR WIFI Telemetry
With the 3DR WIFI Telemetry you just need one transmitter (provided you have a WIFI card/stick in your computer/tablet). Just connect the module to the
TELEM port and it should act as a WIFI station.
essid: APM_PIX password: 12345678
Once connected to the WIFI, it should automatically connect to QGroundControl.