The best way I can describe Hyperwords is a way to super charge your searching via your right-click menu in Firefox. Think of being able to select any word on a page and being able to search for images of that word. You could be looking at this article here, select the word “Mozilla” and go to their Wikipedia entry.
There are so many different word interactions and lookups you can do (plus you can add your own) this is really a power tool that ads much to the Firefox experience. Here is a video demo of some of the features that the Hyperwords extension adds:
To learn more, be sure to check out their handy user guide they have posted, as it covers many of the basic questions you might have about it. Hyperwords is quickly becoming one of my favorite Firefox additions.