Arc Welders can be converted for tig welding by fitting a tig torch that plugs into where electrode holder fitted, the torch has a gas hose coming off it that you fit to a gas bottle with regulator valve . On the tig torch you have a valve that you open and this allows gas to flow, you then scratch the tungsten onto work to initiate arc and you are then tig welding, once finished you pull torch away from work to stop arc and then turn gas valve off.
Its a very basic tig system and suits those who only wish to tig weld rarely.
An actual tig welder has benefits of HF start that start arc when you press torch button so you don't have to scratch start that wears down tungsten, and when you want to stop welding you just release trigger and arc stops & turns off the gas, on the system above you just pull away as MMA welder is 'live' all the time this can leave poor weld at end of weld.
As the price for a HF DC Tig welder is much lower nowadays we would advise to purchase a dedicated DC Tig Welder with HF start and auto gas on / off as once you have purchased the ARC inverter powersource, torch and regualtor etc there is not much difference in the end price.