Natural sources refer to sources that are present naturally and have not been made by human beings. Some of the natural sources of light are:

• Sun: The Sun is the most prominent source of natural light on Earth. The Sun is a star and obtains its energy through the process of nuclear fusion. The Sun's energy gives us light, which is also used by plants to carry out photosynthesis. 
• Other stars: Apart from the Sun, other stars also provide us with light, although a very small amount in comparison due to their large distances from Earth. 
• Lightening: During thunderstorms, we get lightening, which is a momentary source of natural light.
• Volcanoes: Due to the amount of heat inherent in molten rocks, magma and lava provide some light naturally. 
• Certain insects and fish: There are some insects and fish, such as fireflies and jellyfish, that emit light. These are also natural sources of light.

Hope this helps.