It is the negative form of the verb "karte", which means "do".
Here is a breakdown:
* Nahi (नहीं) = "not"
* Karte (करते) = "do" (plural form)
Examples:
* Hum yeh nahi karte. (We don't do this.)
* Vo aisa nahi karte. (They don't do that.)
* Kya tum yeh nahi karte? (Don't you do this?)
The word "karte" can be conjugated to different forms depending on the subject and tense, so the full phrase might vary. However, "nahi" will always be used to make the statement negative.