Allegedly, "indoor air pollution" is a cause of air pollution in the developing countries, created from dirty cooking fires. Although by itself a very small contributor, it has a large impact in places where the majority of population is causing it.
A few years ago China started to invest heavily into lowering their levels of pollution by reducing coal usaging and ramping up renewables, which is already seeing a significant impact vs. those countries that haven't.
PM 2.5 refers to the size of the particle 2.5 microns in size. It is very, very tiny, and completely invisible. These particles have very powerful health impacts as they penetrate all the way into our bodies.