"NSFW" stands for "Not Safe For Work" and is used to designate content that may be inappropriate to other users.
Every user has the option to opt in or out of being presented with NSFW content.
Select what you want to see (or not) by heading to Account > Settings. If you opt out, NSFW images will be visually blocked, but the text of the posts will still be visible.
If you’re posting NSFW content, there is a toggle when you post so that only those who want to see your content can do so.