Protein denaturation is a process in which proteins loose the quaternary structure, tertiary structure, and secondary structure which is present in their native state by application of some external stress such as heating.
In preparing food products this may be a desired situation for example during heat-induced gelation, but in other cases, for example if a potential health effect is diminished, this is an unwanted situation. A classic example of denaturing in proteins comes from egg whites, which are typically largely egg albumins in water.
Fresh from the eggs, egg whites are transparent and liquid. Cooking the thermally unstable whites turns them opaque, forming an interconnected solid mass.