At a molecular level, natural proteins are shaped like coils or springs. When natural proteins are exposed to heat, salt, or acid, they denature -- that is, their coils unwind. When proteins denature, they tend to bond together, or coagulate, and form solid clumps. An example of this is a cooked egg white, which changes from a transparent fluid to an opaque solid.
As proteins coagulate, they lose some of their capacity to hold water, which is why protein-rich foods give off moisture as they cook, even if that are steamed or poached. Fortunately, some heat-induced denaturation is reversible through cooling. This is why roasted meats should be allowed to rest before carving; as the temperature falls, so of the water or "juice"that was forced into the spaces between the proteins are reabsorbed and the food becomes more moist. Denatured proteins are easier to digest than natural proteins.