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

"Information hazard" is just a term for "things you or others learn and then are worse off because of". Examples of information hazards:

- Instructions on how to make a gigaton bomb with processes and materials anyone can acquire and assemble privately.

- A reachable-by-a-country mechanism for blowing up the sun.

- An image designed to temporarily or permanently mess with your vision. Sharing [1] on reddit as a "cool optical illusion" would be mean.

- Probably whatever a computer spits out if you give it a scan of your brain and set it on the task of finding a paragraph/image/movie/whatever that maximizes dopamine production.

- Some people would consider a devastating argument against free will to be an information hazard, predicting that people would then do whatever they wanted because they "couldn't be blamed".

- If you're a hardcore atheist: religions. If you're a hardcore theist who thinks the devil is out to trick you: convincing arguments against belief.

1: http://en.wikipedia.org/wiki/McCollough_effect

You should familiarize yourself with that feeling so you can recognize it later; it's the feeling of falling into an inferential gap. (The thing might be worthless, true, like Scientology. I usually find that resolving confusion is worth my time, though.)