When you're cooking on a grill or griddle, you place this over your burger (or anything else) and it traps the rising heat and radiates it back onto the food. You can use this technique to speed up cooking or, as Shermie does, melt the cheese on burgers without having to leave the burger on the grill too long and have it dry out. Many people put processed cheese on their burgers, and it melts really quickly. But if you put a harder cheese like old cheddar on there, you're going to need to cover it to get it to melt properly. The alternative is to place the cheese on the burger too early and have the red juices run into the cheese.