Comments are a handy way of adding notes to your regular document text and are visible to viewers and collaborators. These can be invaluable for communicating with collaborators about specific parts of the document, as well as making notes about changes you've made or would like to make. When you publish your document as a webpage or print it, the comments will disappear.
To add a comment to a document:
To view the comment history on a thread, including resolved comments, click Comments in the upper right of your document.