Meaning of Android Studio error: Not annotated parameter overrides @NonNull parameter

It’s an annotation, but the correct name is NonNull:

protected void onSaveInstanceState(@NonNull Bundle outState)

(And also)

import android.support.annotation.NonNull;

The purpose is to allow the compiler to warn when certain assumptions are being violated (such as a parameter of a method that should always have a value, as in this particular case, although there are others). From the Support Annotations documentation:

The @NonNull annotation can be used to indicate that a given parameter
can not be null.

If a local variable is known to be null (for example because some
earlier code checked whether it was null), and you pass that as a
parameter to a method where that parameter is marked as @NonNull, the
IDE will warn you that you have a potential crash.

They are tools for static analysis. Runtime behavior is not altered at all.

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In this case, the particular warning is that the original method you’re overriding (in Activity) has a @NonNull annotation on the outState parameter, but you did not include it in the overriding method. Just adding it should fix the issue, i.e.

@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    super.onSaveInstanceState(outState);
}