The Killing Curse is a powerful dark magic spell that instantly kills its target. Voldemort used it to eliminate the Potters, believing that killing Harry would also eliminate the prophecy that foretold Harry's ability to defeat him.
It's important to distinguish between a curse and a killing spell. While curses can inflict harm and misfortune, they generally don't result in instant death. The Killing Curse is a distinct spell specifically designed for fatal outcomes.