Insulation is a measure of how much of the radiation that is coming from the sun actually reaches the earth's surface. This energy depends on a number of variables including: the sun's angle, the length of the day, the slope of the ground, the path length that the radiation has to travel to reach the surface, and the conditions of the atmosphere (clear, cloudy, dusty, etc.)
The intensity is greatest when the sun's radiation is striking the ground at a 90 degree angle - the sun is directly overhead. This condition happens most frequently at or near the equator.
As the angle increases, the amount of energy decreases so that when you are at either the north pole or south pole the angle at which the radiation strikes the ground is almost parallel to the surface and most is reflected back into the air instead of warming the ground.