What do astronomers mean when they say that the sun makes energy by hydrogen burning?

It seems that you have missed the necessary options for us to answer this question so I had to look for it. Anyway, here is the answer. What astronomers mean when  they say that the sun makes energy by hydrogen burning is that, the Sun is fusing hydrogen into helium and releasing energy. Hope this helps.

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.