That means Firefox found a tag in the page which allows you to load a search plugin for that site. If you drop down the search panel, there should be an "Add" item in addition to the usual buttons. (Example attached, from Yandex.)

Normally, adding a site's search plugin would let you use Firefox's search bar to get the same kind of results as using a search box in the page. You probably won't need this very often, but if you find yourself regularly going to a site and then doing a search, it could save a step.

Note: it's possible that the search setup could be somewhat different between the site's search plugin and its pages; Firefox doesn't know, and it can be difficult to compare.

thanks, now i understand