Packets from the Health Department

A representative from the health department stopped by today with a packet of what looked like white silica that is supposed to help prevent the spread of Dengue and Chikungunya fever. He instructed me to poke holes in the plastic and place it in a bucket of water. Thanks to Google translate, I got that far. But I want to know more. What is it? How does it work, and does it work? How much water? Where should I place it? He said the on top of the cupboards was fine but would it better someplace else? Thanks for any information!