The world of Quantum was first discovered in the Ant-Man film in 2015 when Scott Lang shrank to the size of a subatom. He is in a dimension where the law of time and space is very irrelevant. The Quantum World itself is taken from quantum mechanical theory in the real world, which reads if we shrink more than the specified size, universe law will be destroyed. Apart from that, check out the excellent ShowBox app if you need more info about the upcoming movies.
Kevin Feige understood very well that the theory had a great influence on the MCU. In an interview session published by Marvel Studios: in the early 10 years, Kevin Feige has stated that the world of Quantum would have an important role in the future of the MCU.
“At the end (of the film) Ant-Man we follow Scott Lang into the world of Quantum for the first time. We begin to open the first layer and then open all the layers in the Doctor Strange film when we go to the multiverse. So that is a small test that we did.
But, the Quantum world is now a different overall region that we can manipulate with our story. The Quantum World is wider than we imagined, and there are many adventures there, which is likely that we will explore further in the film. “