This question is asking about a particular scale that is used to measure exercise intensity. There are multiple scales that can be used to measure exercise intensity. One such scale is the Borg Scale of Perceived Exertion. This is an incredibly subjective scale, because it measures your exercise intensity by rating how you feel. Things that a person would observe are heart rate, breathing rate, sweating rate, and so on; however, the Borg Scale doesn't use actual measurements. Rather, it is more of a self-check system.
A more common measurement of exercise intensity is to use the target heart rate. This calculation is easy to determine and easy to check in the middle of a workout. Subtract your age from 220 to determine your maximum heart rate. For moderate exercise, take 70% of that max heart rate. The easiest way to check for heart rate during the exercise is to wear a heart rate monitor, and the fitness watch market is flooded with devices across all price points that offer this capability.
A third way to measure exercise intensity is to use "metabolic equivalents" (METs). A single MET is the energy it takes to quietly sit in place. An average adult will use about 1 calorie for every 2.2 pounds of body weight per hour. This means that a 160-pound person burns roughly 70 calories per hour sitting/sleeping. Moderate intensity exercise will get a person's body moving enough to burn 3–6 times as many calories per minute as the person would sitting; therefore, the MET value of moderate intensity exercise is 3–6 METs. Vigorous intensity activities are anything greater than 6 METs. A brisk walk or mowing the lawn are both activities that are considered moderate intensity activities.