Water droplets in the atmosphere react with carbon dioxide gas to form an acid called carbonic acid.
Water is normally pH neutral in other words, neither acidic or alkaline but rain is slightly acidic with a pH of about 5,6. Burning wood or other fuels releases extra carbon dioxide into the atmosphere. When this extra load reacts with atmospheric water, the water becomes more acidic.
Other polluting gases also make rain water more acidic for example, sulphur dioxide and nitrogen oxide. When this water falls back to the earth as rain, it is quite acidic. This is very bad for soils, rivers, lakes and dams that can become very acidic.