The intensity of exercise refers to the amount of effort put into an exercise. Intensity is usually measured by assessing the heart rate during exercise. As a general rule, the intensity of exercise should not exceed certain limits. If monitoring heart rate use the simple equation - 200 minus your age (in years) to estimate the working heart rate you should remain under.
Of course 'listening to your own body' can be just as important and reliable in determining whether or not you are exercising at a sufficient intensity. Accordingly, a feeling of mild fatigue should be your aim immediately following a bout of exercise.