One of the simplest uses for an NPN transistor is as a switch. In this configuration, when the physical switch - which could also be an output from a microcontroller or other digital line - is closed, current flows through the current limiting resistor R1 into the base of Q1, turning on the transistor, and allowing power to flow through it and R1, turning the load on.
Bipolar transistors are current controlled devices, and the resistor R1 on the base of the transistor limits the current flow from base to ground. The transistor will conduct a maximum current from collector emitter equal to the base current multiplied by the transistor's gain, notated Hfe.