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There are 2 answers

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Yash On

Pre-register means register in advance so it means you can register your interest for an upcoming app or game, just as you would register for updates on a new service or a new smartphone. Apps that use the new feature will have a “Pre-register” button where the Install or the Purchase button can normally be found.

Pre-registration doesn’t mean you will have to pay for the app once it comes out; you will simply receive a notification on your device when the app becomes available for download, and it’s up to you if you want to purchase or install it. You can always unregister by visiting the Play Store listing again and clicking “Unregister.”

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Shady Medic On

It means that the developer of the app hasn't released the app for downloading and installing yet, but wants people to know about it. By pre-registering, you show your interest in the app and you will get a push notification once the app is published for download and install. It's something like adding it to a wishlist.