Author Options:

Did I miss something here? Answered

“Basically, an NPN transistor allows electricity to flow when a current is applied to its base and stops the flow of electricity when it is removed. It can let varying amounts of electricity pass through in a direct relationship to the current on its base pin.

Notice when the button connected to the base pin is pressed, the LED turns on. This is because the buttons is connecting the base to power. When the button is not pressed, the 10K resistors is connecting it to ground. This type of resistor configuration is called a "pull-down resistor" because it keeps the pin grounded when not in use.”

Seems you didn’t mention about the LED and resistor in previous paragraph...