Locked mode for Realtime Database consists of these security rules:
{
"rules": {
".read": false,
".write": false
}
}
That will allow non regular client-side SDK access to your data, and thus only allows administrative processes (such as those using the Admin SDK, or project collaborators with access to the Firebase console) to access the data.
If you want your regular users of the client-side application to retain access to the data in a secure way, you will have to implement security rules that control what data they can access and what they can do to that data. This is not a simple yes/no style toggle, but rather it's another part of your business logic - quite similar to what your application code is.
For more on this see:
And many of the previous questions on security rules, such as