The Ecology Dictionary gives us the literal scoop. Yellow rain is a "powdery, poisonous, yellow substance reported as dropping from the air in southeast Asia and found to be the excrement of wild honeybees contaminated by a fungal toxin."
Okay, mildly interesting, right? Well, the real fun begins as we gaze back to 1981-2 when Alexander M. Haig Jr., then Secretary of State, charged that yellow rain was "a Soviet weapon derived from fungal toxins and had been used to kill thousands of people in Cambodia, Laos and Afghanistan." Putting aside Cold War paranoia, some Canadian and Malaysian biologists looked into these charges and eventually reported in the journal Nature that the "swarms of bees, Apis dorsata, need to cool their bodies to lower the temperature of their colonies and protect their developing larvae."
"We've put the phenomenon into a biological context," said Dr. Peter G. Kevan, a biologist at the University of Guelph in Ontario. "In Southeast Asia, the notion of yellow rain as a biological weapon has been pretty much laid to rest. The U.S. Government made a mistake."