You earn "very active minutes" when you wear your Fitbit while doing cardio workouts and high-intensity activities like jogging and running. Note that your active minute count will be lower for activities that are not primarily step-based, such as weight lifting, cycling, and rowing.
By default, you are given a starting goal of 30 active minutes a day based on the Center for Disease Control's recommendation of 20-30 minutes of daily moderate-to-intense activity. You can always change your very active minutes goal to meet your personal needs.
All Fitbit trackers calculate active minutes using metabolic equivalents (METs). MET is a unit used to represent the amount of oxygen used by a body during physical activity; therefore, MET can be used as an indicator for intensity of physical activities. For example, a MET of 1 indicates a body at rest. Your MET value in any given minute is determined by the intensity of your activity based on your speed. Active minutes are earned for all minutes above a certain MET value based on the thresholds we have established. Simply put, active minutes correlate to a greater amount of activity over the course of 60-second intervals.
The more calories you burn the higher your MET value, and therefore the more calories you burn, the higher your very active minutes. When looking at your calorie graph, shown below, the green spikes indicate that you’re earning very active minutes.
Source: https://help.fitbit.com/customer/portal/articles/1020095-what-are-%22very-active-minutes%22-