The Quantum Realm was first introduced in Ant Man, when Doctor Pym tells Scott Land about his wife getting stuck in it due to uncontrollable subatomic shrinking. Later in the movie, we see the Ant Man travel to the realm in a sequence of events that can only be described as fairly trippy. After floating around for a while, he magnifies his size to get out of the realm and into the real world.
The second Ant Man film, Ant Man and the Wasp, delves a little deeper into the Quantum Realm without really establishing any ground rules for the same. We see a magical space and meet the original wasp, played by Michelle Pfieffer. She has aged, however, offers no explanation regarding how she is even alive. Nevertheless, she hops into the Quantum Tunnel with Dr Pym and finally leaves the realm behind. She even manages to fix Ghost, a villain whose major concern is that she is in a constant state of phasing and wants to get out of it to stay alive. Though this villain was slightly annoying, she does offer an insight into what we can expect from the Avengers: Endgame movie releasing this year.
To begin with, Ghost’s cells exist between multiple realms, which is why she is always phasing in an out of reality. When the Wasp heals her, she becomes solid again, showing that no matter how unstable the cells, they can be stabilized and brought back to reality. During the end credits of the movie, we see the team creating a smaller Quantum Tunnel that fits into Scott’s van. While he explores the Quantum Realm, his teammates vanish into dust, because this is when Thanos snapped his fingers. Now, we know from the Endgame trailer that Scott makes it to the Avenger’s facility along with the van. This means that the Quantum Tunnel is an important item which will ultimately create a bridge between the two realities so that our heroes can be saved.
The assumption here is that when Thanos snapped his fingers, the people who died entered the Quantum Realm and the tunnel will help bring them back. This theory is supported by the fact that there have been many images of Captain America and the rest of the Avengers wearing white Quantum suits. We are also guessing that there will be some level of time travel too because there have been many reports of older sets resurfacing for shooting. Time travel would be the only way to restore all the dead people, and not just the heroes. Thanos has already broken the timeline once while taking Vision’s Mind Stone, and this second break in the timeline may give our heroes the edge they need to defeat Thanos.