Hi gman, the relay is made up of 2 parts inside a plastic case, one part is a coil and when a current goes thru' it, it becomes magnetised and this then attracts the other part which is a small lever system that operates a switch, all inside the relay. The electrical contacts of the switch are brought outside the plastic case so they can be connected into a circuit.
So in this circuit the relay coil is powered by the receiver 4.8 volt battery, the quite separate switch in the relay has been wired into the circuit with the 6 voly battery.
When the receiver is turned on the relay coil gets magnetized and this attracts the lever system which turns the switch to the on position and the switch in the relay powers up the 6 volt battery circuit a moment later.
The relay isolates the 2 circuits from each other, so if anything goes wrong in the 6 volt circuit, it does not affect the receiver.
You can see that in a different set up you could have a low voltage operating the relay coil and the relay contacts switching a much higher voltage but they remain quite separate from each other.
Relays come in different types and sizes. You choose one that has a coil suitable for the coil voltage you wish to use and contacts that can handle the current you want to switch.
There are electronic equivalents called solid state switches and these are typically used by having the coil in a digital (5 volt) circuit but the switching part in an AC mains 240volt circuit and the solid state relay keeps them form interacting,
Hope this helps and not TMI,
kind regards Roy