What does @hide mean in the Android source code?

1. @hide is used for things that need to be visible for various reasons but are not part of the published API. They will not be included in the documentation when it automagically extracts the API from the source.

2. You're right, you cannot override it. This is normal, that's by design, since it's marked as final. You should be able to use it, although an editor may not show it to you as one of the choices in whatever intellisense it uses because it's marked with @hide, and you should take note of point 3 below.

3. You should not use it at all since it's not part of the API and the developers can remove it whenever they wish. They would even be within their rights, were they sadistically inclined, to replace it with a function that bricked the device it ran on (though maybe not in a strict legal sense).

Android has two types of APIs that are not accessible via SDK.

The first one is located in package com.android.internal. The second API type is a collection of classes and methods that are marked with the @hide Javadoc attribute.

Starting from Android 9 (API level 28), Google introduces new restrictions on the use of non-SDK interfaces, whether directly, via reflection, or via JNI. These restrictions are applied whenever an app references a non-SDK interface or attempts to obtain its handle using reflection or JNI.

But before API level 28, the hidden methods could still be accessed via Java reflection. The @hide attribute is just part of Javadoc (droiddoc also), so the @hide just simply means the method/class/field is excluded from the API docs.

For example, the checkUidPermission() method in ActivityManager.java uses @hide:

/** @hide */
public static int checkUidPermission(String permission, int uid) {
    try {
        return AppGlobals.getPackageManager()
                .checkUidPermission(permission, uid);
    } catch (RemoteException e) {
        // Should never happen, but if it does... deny!
        Slog.e(TAG, "PackageManager is dead?!?", e);
    }
    return PackageManager.PERMISSION_DENIED;
}

However, we can call it by reflection:

Class c;
c = Class.forName("android.app.ActivityManager");
Method m = c.getMethod("checkUidPermission", new Class[] {String.class, int.class});
Object o = m.invoke(null, new Object[]{"android.permission.READ_CONTACTS", 10010});