Here are some ways to understand and measure the intensity of aerobic activity: Relative intensity vs Absolute intensity.
The level of effort required by a person to do an activity. When using relative intensity, people pay attention to how physical activity affects their heart rate and breathing.
The talk test is a simple way to measure relative intensity. In general, if you're doing moderate-intensity activity you can talk, but not sing, during the activity. If you're doing vigorous-intensity activity, you will not be able to say more than a few words without pausing for a breath.
The amount of energy used by the body per minute of activity. The table below lists examples of activities classified as moderate-intensity or vigorous-intensity based upon the amount of energy used by the body while doing the activity.