A metal collar, usually a slotted steel or cast iron ring 20 to 25 cm across, placed over a gas burner so a round-bottomed wok sits steady and low in the flame instead of rocking on the grate. Traditional woks have no flat base because the whole curved surface is meant to be heated; on a Western stove the ring stands in for the recessed burner of a Chinese range and keeps the heat concentrated on the bottom of the pan.
It makes a difference only on gas; on electric and induction hobs a flat-bottomed wok is the answer, and a ring lifts the pan away from the heat. A wok burner, the very high-output outdoor or built-in ring burner, is a different item sold under a similar name.