Indonesian peatland fires burn in peatlands, wetlands with soils made up of layers of organic matter from decaying plants. Peat forms when waterlogged conditions slow the decomposition of this material.

As described in the news context, peat fires can smoulder beneath the surface and re-emerge, producing especially thick smoke. The fires are contributing to hazardous haze in Indonesia and neighbouring countries. The article says commercial land-clearing burns have been blamed for many of the wildfires.