The title of Sith Underlord was bestowed upon those Sith who were not quite effective enough to merit the title of Sith Lord, but were too old and irritating to keep at the rank of Sith Apprentice. Sith Underlords were often charged with the less important Sith work, such as periodically applying a fresh coat of dust and cobwebs to Sith Temples, turning kittens to the Dark Side, and sharpening Sith lightsabers when they get dull.
Every good Sith Mega-Super-Overlord has at least 12 of these goons working for them. Of course, said Mega-Super-Overlord was taking a "business trip" quite often, so Sith Underlords were the people who did menial chores. Kinda like Jedi Padawans except cooler and older.