The light from the Sun travels out and reaches all the planets. Those nearest the Sun receive the most light and those furthest away, like Pluto hardly get any at all. Earth is 93 million miles from the Sun and light reaching us takes about 7 minutes to travel this distance. As well as going round the Sun most of the planets also spin on an axis - like a child's spinning top.
Sunlight travels in straight lines which shine on half the Earth facing the Sun.
It is day on this side of the Earth and night on the other half which is in shadow. To demonstrate this in a dark room, spin an apple on its stalk while shinning a torch on it from one side. Half the apple will be lit up and the other half is dark. As the apple spins the half that was in darkness moves into light and the half that was in light becomes dark. Today the Earth spins once in just under 24 hours.