In the Marvel Cinematic Universe (MCU), Thanos is an intergalactic supervillian that collected 5 of the galaxy's super powered infinity stones, and used them to snap half of all life out of existence, in spite of the best efforts of the Avengers, a group of superpowered heroes. In the final movie, the Avengers using time travel managed to undo the snap 5 years later bringing everyone snapped out back into existence. For purposes of this debate, it can be assumed there is no impact to time travel.