Resting meat after cooking is the simplest of simple cooking tips to make your meat taste better but the reason why it improves it is buried in food science.
As meat cooks, the muscle tissues firm up and fluid is squeezed out. This moisture moves towards the meat surface, where it evaporates during cooking. When you take the roast meat from the oven, this moisture that is still inside the meat, should be redistributed back through the meat, instead of being released. If you cut a joint straight from the oven, excess liquid pools and rushes out, which equals a dry joint.
Meat will also continue to cook for a few minutes when you take it out of the oven, so there’s more than one reason for resting.
Letting the meat rest allows the moisture to evenly redistribute and reabsorb back into the meat to give a tender juicy piece of meat. You're best off covering it loosely in foil, usually for 10-20 minutes, depending on size.