When the Moon and Sun are placed at opposite directions from the Earth, the sunlight falls on the side facing the Earth. Therefore, the Moon appears as a round shining disc and is known as full moon.
On the other hand, when the Moon comes between the Earth and the Sun, sunlight falls on the side that faces away from the Earth. During this time, the side facing the Earth appears dark and is known as new moon. The sky looks bright during the full moon while the Moon will not be visible during the new moon, making it dull.
Picture Credit : Google