i'll explain it as simply as i can:
the "di" prefix means "two" in chemistry, i.e. dihydrogen = 2 hydrogen
"mono" means "one"
"oxide" is oxygen
"monoxide" is thus "one oxygen" (the second o is removed since "monooxide" would look weird)
combine the two and you get: 2 hydrogen 1 oxygen
in chemical symbols, this is H2O, which is literally just water
"dihydrogen monoxide" (abbr. DHMO) is actually used as a way to make fun of stupid people, by saying things like "DHMO is found in cancer cells, has caused deaths by excessive consumption", etc. in a way that makes it seems like DHMO is everywhere but as to not make it obvious that it's just water; this way, the aforementioned stupid people get scared and try to get it banned, not realizing that it's water, and even a handful of politicians have wanted a ban for the substance, and they're too stupid to realize that it's literally just water while literally everyone with a basic knowledge of chemistry is trying to hold their laughter in — essentially, DHMO is used by intelligent or normal people to make fun of the stupid majority of modern society
