The title for this movie seems pretty self-explanatory, but it can also be a tad confusing. Its full designation is Episode III - Revenge of the Sith. But … it's the sixth movie released in the series.
That's because the Star Wars series started in medias res, with Episode IV - A New Hope being the first one to be produced and released. So, Revenge of the Sith is the sixth movie, technically, but it is the third in the order of how events unfolded.
The revenge to which the title refers is the fact that the Sith come out on top in the end. By converting Anakin Skywalker, the so-called Chosen One to bring balance to the Force, they've officially gained the upper hand over the Jedi and control over the entire Galactic Empire.