that's for the orientation of the drone to the subject. . the icon in the middle is just a form of classification of the subject if it can tell what your trying to track such as a person or car. to use it to control the drone drag the point to where you want the drone positioned to the subject if you don't like where it goes you can control it manually as well to make adjustments and it should follow automatically although with a slight delay. just be careful when you use it because its difficult for the drone to track you efficiently when many or larger objects are present and sometimes might crash even if it has obstacle avoidance. some drones have proven to avoid obstacles better than others such as the
air 3 and some others. the circle with 2 rings just makes it easier to adjust distance from the subject.