In the endgame of the Marvel Cinematic Universe, the Avengers bring back the population lost in the infinity war by Thanos' ""snap"". They retrieve the infinity stones by going back in the appropriate timelines before Thanos collects them. The soul stone (one of the infinity stones) however, can only be gained by a personal soul sacrifice. For this, Natasha sacrifices herself and hawkeye gets the soul stone back to the Avengers. Reuniting in the present, the Avengers place the Stones into a gauntlet that Stark, Banner, and Rocket have built. Having the most resistance to their radiation, Banner wields the gauntlet and reverses Thanos's disintegrations. Tony Stark (Iron Man) uses the Stones to disintegrate Thanos and his army, at the cost of his life. Thus far, in the MCU, Iron Man is considered dead.
But there's a popular theory among fans regarding the use of the soul stone. Since the soul stone cannot be returned and can be used to resurrect and conjure the people who are dead, the Avengers can use that to revive Iron Man.