Light (i.e. energy) is given off from from hot things as they cool down. We're going to talk a lot about LIGHT SOURCES. Our most important light source is the Sun, but I'm sure you can think of many others and we'll discuss them more later.
The emission of light from a light source is usually associated with hot gases. The sun is, of course, an extremely hot object! (There are a few 'cold' sources of light but we won't discuss them here.) The sun is a gigantic nuclear reactor that converts one gas, hydrogen - actually a solid in the sun because of the great pressure involved - into another gas called helium. This produces huge quantities of heat and light. But the sun is rather exceptional in Earthly terms. When we burn fuel there is, thankfully, no nuclear reaction, but a chemical reaction takes place in which the oxygen of the surrounding air combines with the fuel to produce other, usually gaseous, materials. These processes almost always generate heat which is then transferred to the gases and it is these hot gases that give rise to the light.
In approximate terms, we can think of low temperature light as being red, at the long wavelength end of the spectrum, and as the temperature gets hotter, the wavelength tends to get shorter such that at the highest temperatures, light of all wavelengths is being emitted and we talk of something as being white hot.