Active Zone Minutes are counts of time spent in those aerobic based activities. By default, the Fitbit is set up for your goal to be 150 minutes of moderate intensity or 75 minutes of vigorous intensity exercises based on the general physical activity guidelines for Americans¹.

The Fitbit does its best to use your heart rate to determine what intensity you are working in. For every 1 minute the device reads your heart rate as being in the moderate intensity range it will add 1 Active Zone Minute to your total for the day. For every one minute the device reads you are in the vigorous intensity range it will add 2 Active Zone Minutes to your total for the day. 

References

1. Piercy, K. L., Troiano, R. P., Ballard, R. M., Carlson, S. A., Fulton, J. E., Galuska, D. A., George, S. M., & Olson, R. D. (2018). The Physical Activity Guidelines for Americans. JAMA, 320(19), 2020–2028. https://doi.org/10.1001/jama.2018.14854