The principle of progressive overload refers to the principle of continually increasing the demands on your muscles so that you can continually make gains in muscle size, strength and endurance. The body won't change until it's forced to. If you don't continually stress and develop your muscles and strength they won't grow, and they'll eventually reduce in size and strength. The same principle applies to your bones and connective tissues, and to your cardiovascular capability.
The principle can be put into action with a structure of activities to perform - sets, periods, rests, repetitions, frequencies and levels of intensity.
Here's an excellent article: The Ten Rules Of Progressive Overload: Brett Contreras