When you send your supervisor a draft for feedback, they may annotate it electronically using the comment tool. Comments are also useful for you to keep helpful notes for yourself, or to explain where you are up to, to your supervisor. A major advantage of the comments is that they can be hidden, so you can read without the interfering with your reading when you need to. Because of this I much prefer to use comments when making notes to myself than to use highlighting or add temporary text.
This posting gives you some basic instructions which include how to add, delete, and hide comments.