This is about the meet Google Play's target API level requirement. Google Play will require that new apps target at least Android 8.0 (API level 26) from August 1, 2018, and that app updates target Android 8.0 from November 1, 2018.
What does it mean?
The manifest of any APK-file has the code targetSdkVersion (also known as the target API level). It indicates the maximum Android version for the app.
Every new Android version gets changes that bring significant security and performance improvements. Some of these changes only apply to apps that declare support through their targetSdkVersion manifest attribute.
Configuring your app to target a recent API level ensures that users can benefit from these improvements, while still allowing it to run on older Android versions. Targeting a recent API level also allows your app to take advantage of the platform's latest features.
What date and requirements?
August 1, 2018: New apps target at least Android 8.0 (API level 26)
November 1, 2018: App updates target at least Android 8.0 (API level 26)
Once these requirements are in effect, you won't be able to download new APK files from the earlier target API levels to the Google Play Console.
What should I do now?
Right now you don't have to do anything. It's just a warning, so you don't need to change API level right now. When the time comes you will receive an update from us regarding new API level.
But you should be ready. Here you can find the important points you need to know about updating your target API level to meet the Google Play requirement. To proceed, click on your app's current API level:
Earlier than Android 5.0 (API level 21)
Earlier than Android 6.0 (API level 23)
Earlier than Android 7.0 (API level 24)
Earlier than Android 8.0 (API level 26)
But I still can’t update my app because of warnings. How to fix it?
Follow our instructions for updating your app in spite of the warnings: