Original: Comet Shoemaker-Levy 9 (formally designated D\/1993 F2, nicknamed String of Pearls for its appearance) was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of solar system objects.
Simplified: Comet Shoemaker-Levy 9 was a comet that broke apart, and crashed into Jupiter in July 1994. For the first time, people could see how objects in the solar system, outside Eath collided.