Aerobic fitness has a specific technical definition rather than a general meaning. It is the maximum rate at which a person can take in oxygen and use it to produce energy.
The measurement is of oxygen, not effort
The standard test increases workload progressively while the person breathes through a mask that measures the gas going in and coming out.
As intensity rises, oxygen consumption rises with it. At a certain point it stops rising despite further increases in workload, and that plateau defines the maximum.
The value is expressed relative to body mass so that people of different sizes can be compared, which is why it is quoted per kilogram.
The heart is usually the limiting factor
Oxygen has to be breathed in, loaded onto haemoglobin, pumped to working muscle and extracted there. Any of these steps could limit the total.
In healthy people the lungs have spare capacity and muscle extraction is efficient. The constraint is how much blood the heart can move per minute.
Training raises that ceiling mainly by increasing the volume of blood ejected with each beat rather than by raising maximum heart rate, which changes little.
Thresholds matter more than the maximum for most purposes
The maximum can only be sustained for a few minutes. What determines performance in anything longer is the intensity a person can hold below it.
Laboratory testing identifies thresholds where breathing and blood lactate rise disproportionately, marking the points where sustainable effort ends.
Two people with identical maximums can differ substantially in these thresholds, which is why endurance training targets them specifically.
Estimates from wearables work differently
Consumer devices do not measure oxygen. They estimate the value from the relationship between heart rate and pace during ordinary activity.
The estimate is reasonable for tracking change in one person over time, since the errors are broadly consistent, but comparisons between individuals or devices are unreliable.
Anything that alters heart rate independently of fitness, including heat, dehydration, illness and some medications, will move the estimate without the underlying capacity changing.
Why the measure is used beyond sport
Cardiorespiratory fitness is among the stronger predictors of long-term health outcomes in population studies, comparable to established risk factors.
Part of that reflects the fitness itself and part reflects what a low value indicates about underlying disease, which observational data cannot fully separate.
Formal testing carries its own risks in some circumstances and is normally arranged through a clinical or supervised setting rather than undertaken independently.