The sun produces light because it produces energy; light is one form that the energy takes. The sun's energy comes from nuclear fusion reactions at its core, where the temperature is 13.6 million kelvins. This energy travels through the sun's internal layer to the surface, where the temperature is only 5,700 kelvins. From there it radiates into space as visible light.
The main reaction occurring in the sun's core is the conversion of hydrogen to helium. Through a chain of reactions, four protons and four electrons -- basically four hydrogen atoms -- become a single helium atom with two protons and two neutrons. Approximately 0.7 percent of the mass of the hydrogen atoms gets converted to energy in the process, which is equivalent to 4.26 million metric tons per second.
From the Center of the Sun
The energy created by hydrogen-to-helium fusion propagates in the form of photons. Near the core, atoms quickly absorb these photons and convert their energy to kinetic energy, maintaining the high temperatures needed for fusion. The frequency of these collisions and the temperature decrease with increasing distance from the core, however, and at the photosphere, which is the sun's outer layer, some of the photons radiate into space. They are responsible for the light that humans see on Earth.
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