According to the overload principle, exercise must be done at a higher level than usual to bring about various training adaptations. Once the body has adapted to the higher level of exercise it will function more effectively and efficiently.
In other words, your body is going to adapt to exercise. If you start out doing 10 lb dumbbell curls, for example, it may be hard in the beginning and if you are new to weight training, you will see some results. A strengthening of the biceps.
But your body will get used to that weight and if you want muscle gains, you are going to need to increase the load.
The overload can be obtained by manipulating various combinations of exercise frequency, intensity, duration and type of exercise.
Increasing intensity, duration and frequency can be helpful for running, cycling or swimming, and increasing resistance and repetitions can improve strength training.