The Quantum Realm is a location originating from Marvel Comics and a major element in the Ant-Man films. It is a subatomic parallel dimension full of science and magic as the laws of physics are skewed in this plane. It can only be reached by magic, teleportation, or shrinking down to subatomic size.
Scott Lang and Darren Cross also entered the Quantum Realm during their struggle after Scott foiled Darren's plans. Scott managed to defeat Darren by making him fuse with the subatomic particles there, turning him into a quantum anomaly. Scott managed, after much effort, to escape from the Quantum Realm.
Five years later, Scott escapes from the Quantum Realm, and later he, Captain America, Thor, Black Widow, Iron Man, Hawkeye, War Machine, Rocket Raccoon and Nebula use the Quantum Realm to travel to alternate timelines to retrieve the Infinity Stones and undo the Snap. They succeed, and later Captain America uses the Quantum Realm to stay with Peggy Carter in an alternate timeline before returning to the main timeline as an old man. Also, Thor used his trip to retrieve an alternate Mjolnir after the main one was destroyed by Hela.