It's a plugin. Something that loads when you open up the page, so it enables certain features on the site, like Slimbox for example.
There are three levels of "add-ons" for Joomla. They are components, modules and mambots. Mambots tend to be rather minor coding additions to a Joomla website. For instance, they may add social bookmarks to the bottom of a page; they may add style benefits such as special quotation marks; or they may allow you an easy way of inserting streaming videos such as YouTube into the page. You quite often find that a number of components require their own mambots to function properly. Saying that, the WYSIWYG editors are all mambots and are very essential to any Joomla website.