The light from the Sun is a combination of wavelengths, including short ones which look blue to us and long ones that look red.
Wildfires (as well as other events like volcanoes) put a lot of small particles in the air which scatters the light with short wavelengths more than the light with long wavelengths. So the light that reaches our eyes has a lot more 'red' light in it than normal (and a lot less blue) and the Sun appears red.
It is basically the same thing that happens during sunset when the light has to pass through more of the Earth's atmosphere, only more extreme.