Simply set an emitter with fuel and then set the fuel to be ignited by temperature, you can then experiment with the graph and see what it does. At default the first point is origin 0,0 so 0 temp gives 0 ignition 1 temp gives 1 ignition (100% ignition). Move the point along the x-axis, half way along to 0.5. You will now find that the fuel will not ignite and till the temperature is above that threshold. Same with vorticity and turbulence, if you set vorticity to be based on density then the default linear graph basically dictates that as the density rises the vorticity increases in a 1:1 fashion.
TFD is ALL about fiddling and diddling and get a hands on feel for what is doing what.