Header Tags can be a confusing phase when it comes to osCommerce shops. Header Tags, in the original meaning, refers to the H tags (h1 through h6) used in HTML coding. But in osCommerce shops, it refers to the title and meta tags that are placed in the section of a page. The phase was used in the name of an early addon and has morphed into the very popular Header Tags SEO addon and, in osCommerce terms, Header Tags refers to the Header Tags SEO addon. There are other meta tags addons and some have tried to leech off the popularity of Header Tags SEO by using a similar name but they don't come close to comparing with the origiinal.
Details of what it can do for your shop can be found on this page.