The circle of illumination is the line that separates the Earth to create equal parts of day and night. It passes through the poles and allows the entire Earth to have an equal amount of time spent during the daylight and nighttime hours.
The circle of illumination divides the Earth in exactly half. It is a line that is able to be seen from space, and the exact location of the line is dependent on the various seasons. Because the Earth turns on its axis, and the circle of illumination stays in the same general position, the Earth is actually moving around the circle, but it appears that the circle is changing positions. The way that the Earth turns in relation to the circle of illumination is the main determining factor behind how many hours it is light or dark out.
The circle of illumination also plays a small part on the seasons that are experienced. Because the Earth is in a different place on the circle, there are fewer daylight hours. Winter has the least amount of daylight hours, which means that there is less sunlight to help keep things warm. Places on the Earth that are closer to the north and south poles are more affected by the lower amount of sunlight, making winters colder and more intense.