Dihydrogen monoxide is simply a chemical name for water molecules, which are made of two hydrogen atoms and one oxygen atom. The term gained popularity on a mainstream level as part of a hoax that was started in the 1990s that illustrated how facts can be manipulated to prove a point.
The hoax claims that dihydrogen monoxide is a colorless and odorless substance, which water is, that kills a large amount of people via accidental inhalation every year. While water does kill people through drowning, it is a different situation than the statement would lead the reader to believe. In addition, many other points were listed to cause a negative reaction from the general public.