A currently burning wood fire flame, for instance.
The flame itself is a bit of an illustration of what is going on. At the base it is pretty much invisible. This is the area where heat is causing certain materials in the wood to release flammable gases. A little bit above the wood, the gases combine with oxygen and actually burn. There is a great mix of oxygen and gases there, so that level of the flame is often blue. Very hot. This radiates heat in all directions, back into the wood to release more gases. The higher levels of the flame are different colors due to different mixes of oxygen and gases. Smoke is products that don’t burn at that fires temperatures, drawn up by the convection currents. These currents can draw other flammable bits up into the flame as well. Causing pops and sparks as they quickly burn.
Starting a wood fire with large pieces of wood is difficult with just a match or sparks, due to not getting enough initial and continuing heat to cause gases to keep exiting the wood and creating a self feeding flame. So you use small flammable bits, that will absorb a lot of the heat from a small flame and cascade the heat to gas chain, into the larger wood pieces above.
Adding starter chemicals works, due to them almost always emitting flammable gases across their whole surface area. A small flame has readily available gases to feed on.