Firebase Rules: What is .contains()?

I have read through Firebase Documentation and do not understand what is .contains().

Below are the sample rules for Firebase Database in the documentation:

{
  "rules": {
    "rooms": {
      "$room_id": {
        "topic": {
          // the room's topic can be changed if the room id has "public" in it
          ".write": "$room_id.contains('public')"
        }
      }
    }
  }
}

May I know what is this $room_id.contains('public')?

Is it referred to the child of $room_id?