Darth has been the title of hundreds of Sith in the history of Star Wars. But what does it really mean? Originally, Darth was not going to be a group title. Instead, it was the first name of Darth Vader, and Vader was his last name. This is obvious if you are really paying attention while watching a New Hope, because Obi-Wan Kenobi calls Vader Darth.
But still, what does Darth mean? Well, Darth basically means "dark," but, while writing the scripts for Star Wars, George Lucas decided that Darth basically is the word "dark," but he thought Darth sounded more evil than just dark. So, any Sith with the title Darth basically has the title "dark." Example: Darth Vader/Dark Vader.
But there is one more thing. If you didn't know, in the original script for Star Wars, Darth Vader wasn't going to be Luke Skywalker's father, but Vader means father. So why is his name Darth Vader? Well, it's just because George Lucas thought the name sounded cool, and that's really the only reason.