During the braising process, the moist, covered meat is cooked over a "slow and low" heat, and this begins the task of breaking down the tough connective tissues of the meat (the stuff that holds the muscles together and connects them to bone and other soft tissue) into collagen. Over a lengthy time period the heat and moisture build up causing the collagen to melt turning it into gelatin, at the same time causing the meat muscle fiber to tighten and coil-up and expell (squeeze out) its juices until it becomes very dry. The juices mingle with the melted gelatin and fat for a while making a complex mixture of flavors. After enough time passes under these conditions, the muscle fibers give up and relax again and gradually absorbe the desolved collagen (gelatin) and fat, bringing that flavored mixture and moisture back to the meat. All of the fat and gelatin that is trapped inside the meat makes for some very exciting flavors and a very tender texture. Think of something akin to putting a sheet of rich flavored meat silk on your taste buds!
Hope this helps~