High frequency. Since welding isn't lightning, the arc won't start without scratching the surface. This is ok on stick welding and MIG; but if you scratch your tungsten to start the arc with TIG, you might contaminate either the weld or the tungsten you're welding with. This isn't acceptable, TIG is super clean and we want to keep it that way.
This is where HF comes in. Instead of scratching the surface, we just hit the pedal (throttle control) with the torch tip close to the weld (1/8" or so). Now the arc will jump that space riding the HF current and ZAP we have an arc without touching the surface.