When you know your resting heart rate then you also know under how much load your heart is during baseline activity. That is - during times when there is no special load exerted upon it which would require for it to run faster.
If your resting heart rate would be lower for example, then you would probably be able to conclude that the heart needs to beat less times in a minute for it to have the same effect as of the person with a much higher resting heart rate.
Your resting heart rate is an indicator of your fitness level because it directly determines how hard your heart is working at rest and during certain aerobic exercises. The human body's heart can only beat at a maximum of 220 beats per minute but this varies through age. Knowing one's maximum and resting heart could help you in determining the appropriate aerobic exercises that you can engage in.
The best time to measure resting heart rate is when you first wake up in the morning when you are very still in bed because any sudden movements (e.g. light walk) would cause the heart to beat fast.
