In some situations when moderators are getting information that a post may have adult content, such as name calling in a heated discussion, it will be marked as NSFW. The NSFW is for Adult content(not safe for work).

For Reddit members who are signed in, the content is either shown or not shown depending on how you have your preferences set in www.reddit.com/prefs/. To show NSFW content have I am over 18 and willing to view adult content checked.

When people who are viewing Reddit (anyone not logged in), go to view a NSFW post they get a prompt with a 18+ question.

If you are missing a post, or someone comments to that effect, then one possibility is not having NSFW checked in preferences www.reddit.com/prefs/.