When you bite into a piece of meat, how well you enjoy it depends on three things: flavor, tenderness, and juiciness. Even if you're not consciously paying attention to those three qualities, your mouth is.
Flavor and tenderness are determined by cut of meat, what kind of exercise the muscles did, fat content, the age of the animal and cooking doneness. There's a whole range of factors.
Juiciness, on the other hand, is all about the water content of the muscle tissue, which doesn't vary all that much across muscle types. Meat is just naturally juicy. The trick is cooking it so that it stays that way.
Resting Keeps the Juices in the Meat
Mostly, that means not overcooking it. Overcooked meat is dry meat, period.
But the oven isn't the only place roasted meats can lose their juices. It can also happen on the cutting board.
To understand why resting helps keep the juices in the meat, it helps to visualize a piece of meat as a network of cells, each one filled with liquid.
When you put it in the oven, the heat causes the cells to contract, squeezing the liquid out and away from the heat, toward the center of the roast. If you sliced it right then, all that liquid would come sloshing out onto your cutting board.
But if you wait a few minutes, the meat cools slightly and the muscle fibers relax. They unclench, allowing the juices to redistribute throughout the meat. Those cells soak it right back up.
You'll still see a tiny bit of juice when you slice it. But most of it will stay in the meat, which means you'll taste it rather than see it.
Note that this reabsorption of juices won't happen if you overcook your meat. In addition to squeezing out more of the meat's juices to begin with, overcooking causes the protein cells to stay permanently squeezed, which means the juices (or what's left of them) can't flow back in.
Resting Meat Allows It to Cool
This unclenching is a function of temperature. Specifically, it happens at around 120 to 125 F, after you've taken the meat out of the oven and its temperature is on its way back down from 130 to 135 F (assuming medium-rare).
For a medium-sized roast, including whole roasted chickens and roasted turkey breasts, this might take around 20 minutes. For larger roasts, including whole turkeys, 5 minutes per pound is a good ballpark. Steaks cooked on the grill need to rest for 5 to 7 minutes per steak. Figure 3 to 5 minutes per grilled chicken breast.
You can cover steaks or chicken breasts loosely with foil, so they won't get too cold too quickly. But with a bigger roast, you might as well skip the foil. After all, you want it to cool off.
In any case, when you're roasting a bigger piece of meat, you have it a bit easier because you can use a meat thermometer. The kind you insert into the deepest part of the meat and leave in while you roast.
Not only will this help you not overcook your meat, but it will also tell you when it's sufficiently rested.
That's because resting is all about allowing the meat to cool to 120 to 125 F. Simply leave the probe in the meat when you remove it from the oven. When it hits 120 F, start slicing.
When Not to Rest
Additionally, no resting is required if you're braising a piece of meat (which includes anything you cook in a crockpot).
Finally, taking the time to rest a piece of meat can exercise our patience muscle, which is one that might not otherwise get much work.
Fortunately, there's a cooking technique where the meat is slow-roasted at a low temperature, and then allowed to rest. The final step is returning it to a very hot oven just long enough to brown the outside. Once it's browned, you can serve it right away. This can be handy when your guests start chanting for food.