The instructions for roasting a turkey or making roast beef recommend taking the meat out of the oven before it is entirely done, and then waiting an hour before serving. How are we cooking the meat, if it isn't in the oven? what is going on inside the meat?
The doneness of meat is gauged by the internal temperature. Heat reaches the core of the meat product through the process of conduction, from the outer part of the meat to the inside. The outer portions will reach the desired temperature before the center does. When the meat is removed from the oven the heat in the outer parts will continue to move to the center, raising the internal temperature even though there's no longer an external source of heat.
Another reason for "resting" meat, as it's called, is to allow juices to distribute through the entire product. When meat is first removed from the oven or grill liquid will be pooled in such a way that cutting into it immediately will allow a lot of the juice to drain out and make the meat dryer.
An hour seems like a long time unless it's a very large cut of meat. Cooked meats are usually allowed to rest for about 20-30 minutes.