This isn't a part of the story but it means that it's easier to understand what is coming.
There are trillions of universes. They are all identical until a specific moment in time. When a decision is made, one universe takes one choice and the other universe takes the other option. There are now two different realities. The others still exist but you cannot access them easily. It is like a water feature on in a children's playground. You can direct all the water one way but the pool beyond the blockage still exists. The water represents people and the thing that blocks the water is the decision that is made. If you go back in time, you can change the decision and access the other reality. Though there are trillions of universes, the Cursed Child is only concerned with three of them. The first universe is the one that the protagonists live in, where Voldemort is dead, Harry and Ginny are married, and Ron and Hermione are too. The second universe is the universe where Albus is a Gryffindor and Harry separated Albus and Scorpius. The third universe is the one where Voldemort rules. All the universes exist. However, you can manipulate time and decisions made to access others. So, even though Scorpius and Albus 'fix time' in the play, those other worlds still exist. So, everything in this story happened. All of it.
And if all of this went over your head, here is an illustrated version.
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Scorpius and The Final Fight
Fanfiction1 What would have happened if Delphi had won? What would she have done to Albus and Scorpius? How could they fix time? Again? Who could help? Scorpius had no idea at first, but as time began to unfold (in several directions) he realised there might...