Comment Tag: Comment tags are used to display comments within a page's source code. This is often done in HTML and may appear in the document as "<*!*-- your comment here --*>" (without the asterisks).
In the past, placing the comment tags in the source HTML was sometimes a Black Hat SEO tactic. This strategy was often used to give SEOs the opportunity to stuff keywords into those comment tags. This is similar to strategies used today with hidden text on a page; both techniques allow webmasters to increase the keyword density on a page. However, using this strategy will often get a website penalized by search engines and, at best, will do little to nothing to increase rankings.