There are a couple of effects that cause the earth to be hotter at the equator than at the poles, but the most important one is simply the shape of the earth. Because the earth is a sphere, light from the sun hits the surface of the earth more directly at the equator than at the poles, reducing the amount of heating due to sunlight at the poles. The more directly light strikes a surface, the more likely it is to be absorbed and converted into heat.
A secondary effect is that much of the earth's poles are covered in ice, which is better at reflecting sunlight than land or the oceans. Thus areas that have more ice would absorb less sunlight and remain cooler.
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