When all cables, light bulbs and the battery are properly connected, (e.g. battery pole, cable, light bulb, cable, light bulb battery pole) the electrical circuit closes. The potential difference (voltage) applied to the circuit from the battery forces the free electrons to move from the point with low potential to the one with high potential in other words from the negative pole of the battery to the positive one. These moving electrons are called electricity
As electrical current passes through the tungsten wires (filament) inside the light bulbs, the tungsten wires become warm. As the temperature increases, the wires become yellow or white and start illuminating.
The energy thus is transformed from electrical to thermal (heat) and then from heat to electromagnetic radiation (heat and light).
If you look at the light bulbs carefully, you will notice that the light is not as bright as in the previous experiments
(a) simple circuit and (b) parallel connection.
This is because the total resistance of the circuit becomes higher (more lights and longer cables), and hence the intensity of the electrical current (amperage) is lower.
Unscrew one light bulb from the holder, and observe what happens.