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.
During a mission to disarm a Soviet missile in the 80s, Janet Van Dyne was accidentally trapped in the Quantum Realm, where she spent most of life until she managed to escape in 2017. 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.
When the Ancient One was teleporting Stephen Strange across dimensions in order to reveal to him that magic exists, one of the places he traveled to was the Quantum Realm.
Hank Pym later went into the quantium realm to search for Janet after believing he could rescue her. With help from Scott, Hank and Hope were able to finally locate Janet and he brought her back to earth, where Janet reunited with Hope. However, this reunion was short lived as Thanos performed the snap and erased half of the universe, including Hope, Hank and Janet, meaning Scott - who was being sent into the realm - was trapped there for years.
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.
The quantum realm is featured in the episode "What If... Zombies!?". In an alternate 2018, Hank Pym went down to the quantum realm to rescue his wife Janet, but when he finds her, he discovers she has contracted a virus which turned her into a zombie while she lived down there. She then turns her husband into a zombie and together, they return to Scott and Hope as zombies, who flee as the world, including the Avengers, is overrun by zombies, but there are some survivors, including Hope. Some of the survivors like Happy Hogan and Sharon Carter are turned while people like Peter Parker/Spider-Man and T'Challa survive while Scott's head is saved after being attacked.
3 years after the Avengers defeated Thanos, Scott Lang is living with Hope, Hank, Janet and his daughter Cassie Lang, who reveals she has spent the 5 years that everyone was dusted, studying the quantum realm and made contact. However, the entire family is suddenly pulled into the realm and they spend time trying to get out, eventually meeting Kang the Conqueror. When Cassie finds a way out, she, Hope, Hank and Janet get back to the surface, but Scott is tapped by Kang, who nearly kills him until Hope appears and defeats Kang, who is then sucked into a vortex. Hope and Scott embrace and return to the above world.
