This circuit show how two transistors work as a switch to enable light.
When switch i position up then current go to transistor 1 base makes him "enable" (Current flow from colector go to emiter). Current then go to the ground as it's short path then to other transistor. Lamp is off because current go to ground not to transistor 2 base.
When you put switch down then current go to transistor 2 via resistor 2 makes transistor 2 base enable. Current is starting flowing via lamp to the ground via colector - emiter of transistor 2.
