Insolation is a word made up of the parts of three words, INcoming SOLar radiATION. This incoming solar radiation is mainly in the form of light and heat. The angle at which the light strikes the Earth has a lot to do with how and when various places on the Earth warm up or cool down. The two major factors covered in the animation above are the tilt of the Earth's axis causing seasons and the time of the day.
Seasonal changes in insolation are due to one portion of the Earth being tipped toward the Sun and receiving more direct rays while other portions of the Earth are tipped farther away from the Sun and receive less direct rays. This actually causes the seasons. In this picture, notice that even though the sun's rays are evenly spaced, the red area at the upper portion of the Earth is only being struck by two rays where the same are at the equator is being hit by 3 rays. At the same time, the southern hemisphere is tipped more toward the sun so an equal area is struck by 4 rays. In this image, the northern hemisphere is in Winter while the southern hemisphere is in Summer. Be sure to go through the animation at the top of the page to see the entire cycle of the seasons. Keep in mind that the little red regions, acting as light intensity sensors, are of equal area. Also notice that the little image of the Earth orbiting the Sun doesn't change its tilt. The North pole continues to point to Polaris (the North star). Only when we look at the Earth so that the Sun's rays always seem to come from the left, does it appear that the Earth changes its tilt.
More direct rays means more energy is striking the surface of the Earth. Areas with more direct or overhead rays are going to warm up more than areas with less direct rays.
In addition to seasonal changes in insolation, there are also daily changes in insolation. In darkness of course there is no insolation.
The angle of insolation is a measure of how high the Sun is in the sky. As the morning rays strike the ground they contribute very little energy to the Earth. They strike at far too small an angle. As the Earth continues to spin, the Sun's rays strike the ground at a larger angle and therefore are more concentrated. At noon the Sun is directly overhead. Its rays strike the ground at a 90° angle and insolation becomes the greatest. Moving toward evening the rays become more shallow to the ground and again become less concentrated.