Leto, it's a process.
It's (in the case of say, a candle flame) the complex process of wax (solid CnH2n+2 where n is a number between 22 and 27, but basically a connected bunch of Carbon and Hydrogen atoms) being heated through the liquid phase(molten wax), until it becomes so hot, it vapourises (gas) and is able to chemically react with the oxygen (another gas) in the air, giving off light and heat (energy) to form CO2, some soot (C), maybe a little water and some other trace compounds.
The part you see and feel, the light and the heat, are the energy being given off by an
exothermic reaction