I am no expert on the matter, I have just read bits and pieces off the internet and alot of them contradictory so not sure what to believe.
But the view I have at the moment is that the faster your heart beats the more oxygen that can be pumped to your muscles. I read that for average person 80%ish of working HR and less is the aerobic zone [uses oxygen]. Above 80% WHR is anaerobic [no oxygen] and creates lactic acid.
The fitter/more trained your muscles are the higher the WHR percentage rises before anaerobic exercise kicks in. So after training a long time your HR could be 85% of WHR but your muscles have yet to jump into the anaerobic burning zone. And so the fitter you are the higher your target HR [target HR being the rate around aerobic/anaerobic threshold]. Obviously the intensity of exercise for a fit person to reach their target HR is much greater than that of a less fit person.
I read that for this reason, fit peoples HR tend to go much higher than that of a less fit person. But the fitter persons recovery rate will be much quicker [the rate at which the HR drops after exercise].
But during rest, the HR is much lower for a fit person because their heart is stronger and so can pump more blood/oxygen in one pump.