A heat diffuser is used on gas or electric stoves to protect your clay, cooking pots (ollas) or other fragile cookware from direct heat which could damage the pot.
There are different style diffusers. The one shown is lightweight aluminum with multiple perforated layers and a long wood handle. To use, you place the diffuser right over the burner then place your pot on top of the diffuser. This will keep the heat lower so it will affect your cooking time.