All objects emit radiation. The amount of radiation, called its intensity, and the range of wavelengths it contains both depend on the temperature of the object. As an object gets hotter, the intensity of the radiation it emits increases and the peak wavelength becomes shorter.
An object also absorbs radiation from its surroundings. If a body is at a constant temperature, it is absorbing radiation at the same rate as it is emitting radiation, so the two rates are equal. The temperature of a body increases when it absorbs radiation faster than it emits radiation. It falls when it emits radiation faster than it absorbs it. A cup of tea left in a cooler room cools down until it reaches room temperature. At that point the two rates are equal.
The temperature of the Earth is set by the same balance. The Earth absorbs radiation from the Sun and emits radiation into space. Some radiation from the Sun is reflected into space, for example by clouds and ice, so it is not absorbed. The temperature of the Earth depends on the rates of absorption and emission and on how much radiation is reflected.
