If you connect a solenoid-wound inductor to an AC source, an alternating magnetic field will be generated about the coil which will build to some maximum strength in one direction, will fall to zero, will build up again in the opposite direction, will fall to zero, and then the cycle will begin anew.
Now, if you place another coil close to, and with its axis not aligned orthogonally with the one which is generating the varying magnetic field, the field will pass through the second coil and will induce/generate a voltage difference which will appear across the ends of the wire comprising the second coil.
If those ends are not connected to anything, the voltage induced in the coil will have no work to do, hence there'll be no current in the secondary coil.
If the ends are connected together, then charge will flow and there'll be current in the secondary.
Notice that nowhere has "ground" been mentioned, and yet everything worked, OK?
Now let's move on...
Say we have three of the dual solenoid circuits, one which outputs 1 volt, AC, one which outputs 2 volts, AC, one which outputs 3 volts, AC, and that we've connected one end of their secondary coils together and have let the other ends float.
If we connect one lead of an AC voltmeter to the place where the three ends were connected together, then we can use the other lead to probe the floating terminals and we'll find that one of them outputs 1 volt, another outputs 2 volts, and the other 3 volts.
The place where they're connected together is called "ground", by convention, since it's where the measure of everyone's tallness was measured, in antiquity and to this day.