The heart rate that indicates your fitness level is the

The heart rate that indicates your fitness level is the Resting Heart Rate.

Explanation:

The statement from Welch says that when we practise more and do an exercise harder, then the heart rate at rest would be low. This is the main reason for considering resting heart rates as a good indicator to one's fitness level. The normal adult heart rate at rest is 60 to 100 bpm (beat per minute).

In general, a lower heart rate means a calmer heart rate and good heart health. For example, for a well-trained athlete, a normal heart rate can be closer to 40 bpm. Our resting rhythm, the measure of how often the heart beats every minute at rest.