It should be meaningful to the student and motivate the student

Excellent question, Nikola. 😀 In my opinion it is very hard to pinpoint (single out) just one quality of a teacher that is the most important characteristic in helping a student to improve. One of the most important qualities must be the teacher's ability to create a positive environment where every student feels safe to make mistakes. This itself can only happen through the teacher's skill in building a positive rapport with each of their students. So, in conclusion I would say the most important quality is having very good interpersonal skills. The next most important characteristic must be possessing excellent communication skills. This is only my personal viewpoint; others are free to agree/disagree as they so choose. 😉