Collagen determines, to a large extent, whether cooked meat ends up tender or tough. It is also the determining factor in how long you should cook a given cut of meat. Collagen fibers are the biological equivalent of steel cabling, forming a mesh that holds bundles of meat fibers together. Proper cooking unravels the cable-like structure of collagen fibers and dissolves them into juices, transforming the tough collagen into tender gelatin.
In order to unravel collagen fibers, you must heat them. Heat causes the fibers to shrink, and the contracting mesh squeezes juices out of the meat. The hotter the cooking temperature, the more collagen mesh contracts, and the more juices are lost. If you cook the meat at lower temperatures, fewer of the collagen fibers shorten at any given point in the cooking process, so the mesh constricts the meat less. This is why meats retain more of their juices when cooked sous vide. But at lower temperatures, more time is needed to shrink, unravel, and dissolve enough of the collagen fibers to make the meat pleasantly tender.
Adapted from Modernist Cuisine