The switch.

Generally we use a switch to turn something off or on such as a light.

But how is that accomplished?  You need a power switch, power source, and a power drainer like a light bulb.  Normally the switch is open so that no current can reach a light bulb as a full circuit

So if we can close the circuit so cuurent can reach a light bulb as a full circuit, the light will turn on.

Actually we can do sort of the same things with an electronic switch or transistor, If there is not power to the junction of the transistor then the light or led's will not light up.The advantage is that no traditional switch has to be touched.

Then if we add power to the base of the transistor or led's they light up.

So we can tell a computer or microcontroller to turn on the power to light am led.