Kuroko are men dressed all in black (even with veils over their faces) that often appear on the stage and hiding behind the scenes during Kabuki theatre performances. Their jobs are as stagehands meant to take care of props and act as assistants to the actors.
When they appear on the stage, the audience is supposed to treat them as invisible and focus only on the actors. Because of this, Kuroko move with stealth and small movements that make them reminiscent of ninjas. The name Kuroko literally means "black child."
Kuroko are also known to appear in bunraku, which is a traditional Japanese puppet theatre.
Relevance to Urusei Yatsura
You may have noticed Ryoko Mendo is always surrounded by kuroko. She has a legion of these kuroko under her command as servants. Not only do they look like kuroko, but they behave just like them too, as if the entire world was the stage and Ryoko was the central actor. They act as if they're supposed to be invisible, but they're obviously not.