principles of weight training, overload states that a greater than normal amount of stress or load on the body is required for training adaptation to occur.
Once overload has been achieved, the body will adapt to this stimulus, which requires you to change the stimulus yet again to progress in your training goals, whether that is increased strength, size or endurance.
Muscles may be overload by adding additional weight, increasing reps or increasing sets (volume), as well as slowing the tempo of an exercise. Maintaining the same level of stimulus will maintain previous gains (but not increase them) and removing or pulling back on the level of stimulus, will cause regression in strength, size or endurance.
The act of continuously overloading muscles is called progression.