WiFi telemetry enables MAVLink communication between a WiFi radio on a vehicle and a GCS.
WiFi typically offers shorter range than a normal telemetry radio, but supports higher data rates, and makes it easier to support FPV/video feeds. Usually only a single radio unit for the vehicle is needed (assuming the ground station already has WiFi).
PX4 supports telemetry via UDP and Wifi. It broadcasts a heartbeat to port 14550 on 255.255.255.255 until it receives the first heartbeat from a ground control station, at which point it will only send data to this ground control station.
Compatible WiFi Telemetry modules include: