Thanos is a Marvel comic book character. In Marvel’s Cinematic Universe, Thanos becomes omnipotent by collecting the six Infinity Stones, which are mounted in a gauntlet. Motivated by witnessing the demise of Titan, his home planet, due to the exhaustion of resources, he comes up with a drastic solution.
With a snap of his fingers, he destroys half of all life in the entire universe. To him, this is an act of mercy; a way to restore balance.
After succeeding in his mission, The Avengers – a gang of various Marvel superheroes including Iron Man, Black Widow, Captain America, and Captain Marvel, use quantum-time-travel to undo Thanos’ action.
When he nevertheless regains the Infinity Gauntlet, Thanos instead intents to use it to destroy all life, considering the resistance against his original action as ultimately equally destructive.
Of course, The Avengers prevent this from happening.
The Infinity Stones
Mind | Yellow: sulfur
Power | Purple: amethist
Reality | Red: quartz
Soul | Orange: quartz
Space | Blue: apatite
Time | Green: calcite
Further reading on The Thanos Solution
Is Thanos Right? (The Observer)
Why Thanos didn’t double resources. (UX Collective)