NSFW is an abbreviation for words like Not Safe For Wife, Not Suitable For Work, but mostly accepted as Not Safe For Work.
It is an internet slang that is often used to describe online posts that are mainly associated with heavy nudity, sexual, heavy profanity and other things that could often link with pornography. Most of the time, NSFW is used for a fandom of some sort when people want to discuss and share the sexual and porno side of the fandom.
Short for Not Safe For Work (or Not Suitable For Work), which includes things like: Pornography, gore, fetishes and other shit. If any of your co-workers or your boss finds you watching NSFW, it's gonna be embarrassing and you'll get fired. Watch it at home at your own privacy.
Worker: Huh? This is interesting... "Anime girl has a fun time"
*sees hentai and hears moaning*
Worker: Oh shit! I better clear this NSFW crap and clear the history before anyone sees it!
Boss: DID I JUST HEAR MOANING?!?
Worker: No sir, it was just "business" ;)
Not Safe For Work. Used to describe Internet content generallyinappropriate for the typical workplace, i.e., would not be acceptable in the presence of your boss and colleagues (as opposed to SFW, Safe For Work).