Comments in Word allow you to provide feedback in a document without changing the text or layout. You can place the cursor in the content or select content (text, images, tables, etc.) and add a comment about that part of the document.
When you add a comment to some text in a document and then click away, you may notice that there is no line connecting the comment to the text it corresponds to (as shown on the image below), until you move your mouse over the comment or put the cursor in, or select that text, again. If you want to be able to see at a glance which comments correspond to which text, without having to hover your mouse over each comment, you can easily add the lines back to the comments.