The NPN Transistor acts as a switch or amplifier. With no current at the base, there will be no current flow from collector to emitter. However, a small amount of current to the base will allow a large current to flow through the collector and emitter.
The NPN Transistor module contains an NPN Transistor and a resistor in the wire leading up to the base to protect it. There are also to diodes to protect the transistor by ensuring that current does not flow into the transistor the wrong way. When there is even a small amount of current at the base foot, there is a current large enough to turn on the transistor, allowing a large current to flow through from the collector to the emitter. This allows you to show a small change at the base to be shown as a large one through the emitter and collector.