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.
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.
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.