Since the conclusion of Avengers: Infinity war, ending up with an apocalyptic break, Marvel fanatics have spent the following months trying to figure out how the heroes are going to save the day. As we all know that things have to end well, hence, the heroes will return, so it's not a matter of whether they'll reverse the snap or not, it is the matter of how they are going to do it.
Most of the things in comic books, suggesting the ways in which the Avengers could return are needlessly complicated—like Quantum Realms, time loops, etc.
But the explanation essentially involves Doctor Strange casting a time loop spell on the time stone, in this theory, Avengers: Endgame begins with Thanos back on his home planet—restored to its former beauty—when it abruptly reverts back to the time before the snap.
It’s a part of Doctor Strange's plan. We witnessed in DOCTOR STRANGE that he can cast a spell on the Time Stone to impose an infinite time loop, symbolized by a green energy ring revolving around his left wrist. Irrespective of what happens, it will always reset to the point in time when the spell was casted initially. The Stone and the spell on it keep working, even after Doctor Strange gets killed. This point is important, and ties in with Strange's later reminder to Ebony Maw that spells he casts on the Time Stone, will persist even after he dies. When he lastly gets Dormammu to concede and let loose the spells, the energy ring on his left wrist isn't visible again.
So basically, during the fight on Titan, Doctor Strange was utilizing that time to furtively cast his spell on the time stone before passing it over to Thanos. Strange needed a convincing way to surrender the Time Stone to Thanos, devoid of Thanos catching onto him. When Doctor Strange hands over the Time Stone, he had already casted the time loop spell on it. At some point in the future, the spell would rearrange time to the moment Strange casted the spell, on Titan previous to the Snap. The time loop is factually the "endgame" Strange refers to, as it was with Dormammu.
The key factor here was Doctor Strange's left arm. As we know that, the left arm glows when it uses the spell—something that Doctor Strange was hiding after he cast it on the time stone.
So, while Thanos is captured in that time loop cast by Doctor Strange on the time stone—that's the time when the Avengers will have to regroup, travel into the past and stop Thanos. That's where the time journey in the movie begins, driven by Ant-Man, who has previously discovered how to use time vortexes in the Quantum Realm to get where and when you want to go. They can use the Quantum Realm to travel in both space and time.
It’s hard to think the fact that, how these insights are going to be explained in the two hour movie, but as viewers, we all want our heroes back and fight the evil Thanos to bring peace to the world.