The Buy Now buttons in an oscommerce shop are coded in a way that allows the search engines to follow them, which means a search engine can add something to the cart. This will cause what are known as Session ID's, or SID's for short, to be added to the link the search engine stores. This can cause invalid liks in the search engine listings. By installing this package, the Buy Now buttons are changed to a form that the search engines can't get by, thus eliminating the problem.
Also see the SID Killer contribution since it works in conjunction with this one.
Please note: This contribution is outdated and no longer needed for most oscommerce shops. It is needed for earlier versions though so if in doubt, please contact us so we can determine if you need it or not.