Progressive overload is the gradual increase of stress placed upon the body during exercise.
This includes the cardiovascular system and muscular skeleton system.
Progressive over load relates to volume, intensity and rest periods. To progressively overload the calf muscles in the gym after injury one would start with low reps and no weights slowly increasing the reps and weight over a set period of time.
Here is an example of an Achilles/calf rehab program over 6 week period using progressive overload, You can see that each session builds upon the previous session using small increases in load each day.
Progressive overload is applied to all methods of training and can be applied to all forms of exercise and is best used to prevent injury and allow the body the adapt to new training loads. All training programs should be designed to progressively overload.