positive relation between the teacher and the students

Feedback is how you comment (in speech, writing or just through your body language) on another person's behaviour. Positive feedback is, for example, saying "Well done", or "Good" - negative feedback would be something like "That was terrible". Corrective feedback involves pointing out to students where they went wrong so they can improve. These days it is always considered good to find ways of giving feedback that is positive rather than negative - e.g. instead of telling someone that they came bottom in a test (negative feedback) you tell them that they did better than last time, or that they answered question X very well (even if they got all the others wrong).