Basically a circuit loop, one with energy source, and a load, is completed so current can flow.
Out from one battery electrode, thru the switch, then the load, and finally back to the other battery
electrode completing the circuit.
When switch is open its is an infinite resistance connection. And from ohms law we know
I = E / R, so if R is infinite then no current flows. If switch closed R ~= 0 (but load has some
resistance) so current flows.
Regards, Dana.