When cooking over fire you don’t always want the food directly over the flame. many cuts of meat and other foods need to cook slowly, more like baking or roasting than grilling, and for this we use indirect heat.
Indirect using a gas grill:
Heating indirectly with a gas grill is straightforward. Get all burners going to heat up the grill with the cover down, and once heated, turn off all burners but one. With my tube burner grill I leave on the one at the far left, and turn off the other two. This provides enough heat to keep the closed grill at about 300F, and anything on the right half is completely away from the flame. On my smaller H burner grill, there are only two burners, so one half is hot, leaving slightly less than half for indirect heating – though I can get higher temperatures because it is a larger burner. Interestingly, I find that the warming rack above tends to be hotter than the grill (heat rises, remember?), which is the opposite of cooking over direct heat.
Indirect cooking over charcoal is much the same as over gas, but requires a little more hands-on work. What you want to do is get the fire going at one side of the grill, and place the meat on the other. You also want to raise the grill up as high as it will go, to keep the food from cooking unevenly. Many grills, like Kamado style cookers, have heat deflector plates for low and slow cooking. These are thick ceramic plates that are placed directly over the coals, so the heat has to flow around them, allowing you to cook indirectly over the entire cooking surface.
Cooking over indirect heat is fairly simple and intuitive, and the more you do it the more you will learn the idiosyncrasies of your grill, and it will become completely second nature.