What does Nietzsche mean when he says "And if you gaze long into the abyss, the abyss also gazes into you"?

When Nietzsche says "And if you gaze long into the abyss, the abyss also gazes into you," he means that when you deeply contemplate darkness or evil, it can also affect and influence you in return. It suggests that prolonged exposure to negative or destructive thoughts can have a profound impact on your own psyche.