The significance of resting heart rate in assessing fitness levels lies in its indication of cardiovascular efficiency and overall health. A lower resting heart rate typically suggests a higher level of cardiovascular fitness and improved heart function, as the heart does not have to work as hard to maintain circulation. Tracking changes in resting heart rate can help evaluate the effectiveness of a fitness regimen and indicate recovery levels after exercise. Athletes often have a lower resting heart rate, reflecting their enhanced heart efficiency and fitness status.