NSFW is content that is Not Safe For Work. These categories include Nudity, Pornography, Profanity, Violence and Gore, Race and Religion. If you post this type of content, we ask that you tag it properly. If you don't, you could be reported and receive a strike.
To tag a post as NSFW, click the E tag in the icon tray of the compose field
...Or on mobile, click the cog icon to access the Post Options menu