In an academic setting, you are not expected to take a text at face value, but rather to evaluate and critique it. Your evaluation sometimes means giving feedback directly to the author, which often takes the form of writing a peer review. Being able to evaluate a text depends on your critical reading skills; learning how to translate that evaluation into a written response depends upon understanding the difference between critiquing (which is the point of feedback) and criticizing (which is not the point of feedback).
Whereas criticizing means that you focus on pointing out things you consider negative in an essay, critiquing involves providing a reflective evaluation that brings up both strengths and aspects that could need revision.
The exact focus of your feedback may vary according to the assignment at hand, but useful constructive feedback is always clear, focused, and coherent. Be sure to mention both the strong points of the text, as well as points that could be improved. Make sure you give specific examples from the text so as to make it easier for the writer to understand your comments.