Acid rain refers to rainfall which is unusually acidic, harming plants and fish and causing erosion to buildings and man-made infrastructure.
Typically, acid rain is a result of polluted air mixing with water in the atmosphere producing a reaction that increases the acidity of the rainfall.
Pure water has a neutral pH of 7 meaning it is neither acid nor alkaline. Normal rainfall has slightly acidic pH as it has reacted with carbon dioxide and other naturally occurring elements in the atmosphere. Acid rain in heavily industrialised areas has been recorded with a pH of less than 3 in extreme cases. That's about the same as vinegar (acetic acid). Nasty.
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