A light switch is just that a switch it either completes the active side of the circuit when on or breaks the circuit when off
Or to put it another way your light fitting itself should have three connection points one for neutral an earth connection and an active connection , the active comes via the switch , when it is on the circuit is complete and light is on , when it is off the active line is open and the circuit is broken and the light is off
However your electrician will proberly be able to show better you when he/she fits them![]()