I'm just exploring the new Firebase Firestore and it contains a data type called reference. It is not clear to me what this does.

• Is it like foreign key?
• Can it be used to point to a collection that is located somewhere else?
• If reference is an actual reference, can I use it for queries? For example can I have a reference that points directly to the user, instead of storing the userId in a text field? And can I use this user reference for querying?