Its a fitness test that is available in models f11, and f55
and maybe some of the higher cycling models as well.
heres the description from the site:
Polar Fitness Test™ with OwnIndex®
Measures your fitness level in 5 minutes.
A fitness test that measures your aerobic/cardiovascular fitness at rest in just five minutes. The result, Polar OwnIndex, predicts your maximal oxygen uptake (VO2max).
With the Polar Fitness Test you can measure your aerobic fitness by yourself, automatically and without any exertion. No other equipment other than a heart rate monitor is needed. The Polar Fitness Test is as accurate in predicting the VO2maxas any sub-maximal fitness test.
The test is based on gender, age, height, body weight, level of physical activity, heart rate and heart rate variability at rest. The OwnIndex ranges usually from 20 to 95 and is comparable to VO2max, the golden standard of aerobic fitness.
OwnIndex is most meaningful when following changes in your fitness over a long period of time. Find out how fit you are for your age and gender by comparing your result to the global references listed in the Polar user's manuals.