The prequels to the sequels in the Star Wars saga are The Phantom Menace, Attack of The Clones and Revenge of The Sith. They chronologically serve as the lead-in story to the original Star Wars movie, A New Hope.
The Phantom Menace was released in 1999 and was seen as the beginning of the prequel trilogy, bridging the gap between the time of the Old Republic and the rise of the Empire.
It depicted a young Anakin Skywalker and his journey under the tutelage of Obi-Wan Kenobi, as the two attempted to stop an emerging Sith Lord.
The second prequel in the series, Attack of The Clones, came out in 2002. It shifted its focus to the clash between the Republic and the Separatists, as the Jedi fought to prevent a war that would ultimately lead to their own downfall.
The third and final prequel in the series, Revenge of The Sith, was released in 2005. It's set three years after Attack of The Clones and details the shift in the galaxy from the Republic to the Empire, chronicling Anakin's rise and fall from power.
The prequels tell the story of Anakin Skywalker and his rise as the chosen one who would bring balance to the Force. They are the necessary stepping stones to the original 1977 classic, and set the stage for the further adventures in the Star Wars universe.