It's a concept coined by Nick Bostrom. Basically, some information can be harmful. As a simple, non-controvertial information hazard, imagine you learned an identity of a covert assasin of a foreign government, and you start telling about it to people around you. Don't be surprised if not only you drop dead, but the people you interact with as well.

http://www.nickbostrom.com/information-hazards.pdf

http://wiki.lesswrong.com/wiki/Information_hazard