Telemetry Radios can (optionally) be used to provide a wireless MAVLink connection between a ground control station like QGroundControl and a vehicle running PX4. This makes it possible to tune parameters while a vehicle is in flight, inspect telemetry in real-time, change a mission on the fly, etc.
PX4 supports the following telemetry radios:
See Pixhawk.org > Radio Modems for more information.
WiFi telemetry has shorter range, higher data rates, and makes it easier to support FPV/video feeds. Also you only need to purchase a single radio unit for your vehicle (assuming the ground station already has WiFi).