Diyu is the equivalent of hell in Chinese mythology. When someone dies, their soul's first stop is Diyu. There, they are judged, and a punishment is handed out. Unlike in many other cultures, people are not consigned to Diyu forever. The amount of time spent there depends on how much they sinned in life. If you were a relatively good person in life, you won't have to spend much time in Diyu before being freed and reincarnated. But if you weren't? You're in for a long, painful journey. Because in Diyu, you can feel pain, but you can't die. And whatever damage inflicted upon your body after a round of torture is erased before the start of the next round, ensuring that you feel every whip, cut, burn, and disembowelment.
