Harry Potter was the only person in the Wizarding World to survive the Killing Curse twice in his life. The great mystery of his survival the first time was solved by Albus Dumbledore when he realized that Harry was protected by his mother's love. The second incident was a bit more complicated because Harry was no longer protected but still beat death. A fan theory suggests that Harry was able to survive the curse again because he had become the Master of Death.
Harry Potter and the Half-Blood Prince introduced the idea of Horcruxes, which are parts of the soul that are split to keep a person alive. In Harry Potter and the Deathly Hallows: Part 2, Harry realized that he contained one of the pieces of Voldemort's soul, making him a Horcrux that would have to be killed for Voldemort to die. He decided to sacrifice himself and met Voldemort in the Forbidden Forest, where he was hit with the Killing Curse. Harry appeared to die since he reunited with Dumbledore in a mystical white void, but he was given the choice to come back to life. The fan theory by Reddit user drcompo02 suggests that Harry survived Voldemort's second attempt at killing him because he possessed all of the Deathly Hallows at the time. The Deathly Hallows were made up of the Elder Wand, the Resurrection Stone and the Cloak of Invisibility.
Harry was gifted an invisibility cloak in Harry Potter and the Sorcerer's Stone, which used to belong to his father. Invisibility cloaks usually faded after a few years because they were made with Demiguise hair, but Harry's had always worked perfectly fine. It was believed that Harry's cloak had been passed down from Ignotus Peverell, who had received the cloak from Death himself, at least according to the stories. Harry received the Resurrection Stone from Dumbledore, which was hidden inside the Golden Snitch that he caught in his first Quidditch game. He unintentionally became the master of the Elder Wand when he disarmed Draco Malfoy. The Elder Wand belonged to Dumbledore but he was disarmed by Draco in Half-Blood Prince. Harry later disarmed Draco in Malfoy Manor in Deathly Hallows: Part 1, making him the new master of the wand. This sequence of events made Harry the owner of all three Deathly Hallows at the time that Voldemort tried to kill him.
The general understanding of Harry's survival is that the Killing Curse only destroyed the piece of Voldemort's soul and not Harry himself. The theory states that the only other living being that was a Horcrux was Voldemort's snake Nagini, and she wasn't resurrected after she was killed because she wasn't the Master of Death. If Harry was alive because the curse only killed Voldemort's soul, then Nagini should have come back to life too. Since there are no other known humans that were Horcruxes, there's no way of knowing if they would survive a Killing Curse as well.
The theory further speculates that Harry might be immortal because he owns all of the Hallows. However, the only Hallow that remained with him after the Battle of Hogwarts was the cloak. Harry got rid of the Elder Wand by breaking it in the film and returning it to Dumbledore's tomb in the book. He also dropped the Resurrection Stone in the Forbidden Forest before meeting Voldemort and decided to leave it there. It wouldn't be possible for him to continue to be the Master of Death without all three artifacts.
This theory seems somewhat possible since the lore of the Deathly Hallows does state that the person who owns the Hallows is the Master of Death. Harry may have been given the choice to go on to the afterlife or return to the world of the living because Death answered to him. Nagini's death might not support the theory, however, since she was killed by sword and not with magic.