It's not a prequel, even though it's set in the past.

There had been rumors, now there is fact: Harry Potter and the Cursed Child will come to the British stage next year, and will tell a story of Harry Potter before he went to Hogwarts. JK Rowling did not write the play, but she has provided the story and worked closely with those involved. That means this will be part of the surprisingly fast expanding Harry Potter canon. 

When Harry Potter and the Deathly Hallows came out I really thought we were done with the boy wizard; it wasn't that I wanted him to go away - I love that world! - but rather that JK Rowling seemed quite content in leaving Hogwarts et al behind. But in the last couple of years she's been really ramping up the Potter stuff, and what's nice is that she is so unbelievably rich the only reason she's doing it is because she wants to do it (or perhaps, in the case of Fantastic Beasts and Where to Find Them, wants to make sure other people don't screw things up. Same difference). What's more, she has so far been really canny in how she expands the world - there are some short stories on Pottermore that give us glimpses of older versions of the characters but mostly she's filling in the past and other areas of her Wizarding World.

Harry Potter and the Cursed Child is set in the past, but don't expect it to be a prequel... according to Rowling, anyway. The Daily Mail seems to think it's mostly about Lily and James Potter, but details are scarce right now. Rowling did say on Twitter that those asking why The Cursed Child wasn't a novel would find out that the stage was the perfect way to present this story.

You know what's cool about this? Harry Potter and the Cursed Child could be the sort of thing that introduces a lot of people to theater. Many, many Harry Potter fans with no experience of the magic of live theater will go out of their way to see this latest chapter in the story of the Boy Who Lived and could find themselves under its spell. That would be amazing.