A flashlight involves the simplest known type of electrical circuit.
When the circuit is closed, making a loop, an electric current circulates through it. This involves the movement of electrical charges (electrons) that react to the voltage created by the batteries. This voltage, also known as the potential difference, forces the electrons to move from the negative pole to the positive one. The filament of the bulb is made of a high resistance material, like tungsten. The filament heats up to the point of becoming incandescent, thus generating light.
Click to switch on the flashlight.