Put simply, feedback is information about how we are doing in our efforts to reach a learning outcome. Some examples of feedback include:
"The first few paragraphs kept my full attention. The scene you painted was vivid and interesting. But then the dialogue became hard to follow. As a reader, I was confused about who was talking, and the sequence of actions was puzzling ."
"Each time you swung and missed with the bat, you raised your head so you didn't really have your eye on the ball. On the one you hit hard, you kept your head down and saw the ball."
Teachers should try to carefully observe and comment on what they have seen. We should aim to provide actionable information through our feedback. However, too much feedback may be counterproductive: it's better to help the student focus on just one or two key elements at a time.
As teachers, we should also be wary of mixing praise with feedback. It is perfectly reasonable to motivate students during a learning activity (e.g. "I can see you're working really hard on this challenging activity....") However, some research has suggested that comments such as "well done" or "good effort" (i.e. value judgements) included within oral and written feedback can take the focus away from comments specifically related to a learning goal(s). Feedback might relate to the 'positives' observed (e.g. "The scene you painted was vivid and interesting...") but such comments are different from value judgements, such as "fantastic work!"
It is important to note, too, that there is a difference between feedback and advice. Advice will usually follow descriptive feedback. As teachers, we can be quick to give advice about how to improve before learners have had time to process the feedback and consider their own 'next steps'. Therefore, you might consider including written or oral questions to prompt further thought and thus student-directed action towards meeting the learning outcome.