An add-on (the word was suggested by petitlouisc) is a term that means a product “extra”, something that adds value to a product or service to benefit the consumer. This means it’s good news for customers. Internet users were quick to adopt this term to mean an extension to a piece of software made available to customers in order to enhance the functionality or performance of the software, or allow the life of the software to be prolonged at the lowest cost. Lovers of computer games are always on the lookout for the add-ons the games publishers bring out to enhance their play. In a way, these are extensions that can be added on to the basic version of the game: they let you go to a higher level of difficulty, or give you new weapons or new game plans.
However, add-ons are not confined to video games, as the publishers of conventional software also produce them. Firefox, the browser, offers a catalogue of add-ons, like Search Preview, which you add to your search engine. If you Google the words “video games”, you won’t only get results (lists of sites and blogs, etc about video games), but you will also see how popular those sites are in the Google rankings and get a preview of their home page.
It’s not just software publishers that use add-ons. In the computer world, they are used to mean any addition or supplement, including to hardware. To take one example, the memory card you use to increase the memory of your computer is an add-on.