"Not Safe For Work." It's a tag that people put on photos (almost always pornographic or of naked people) to warn people that they're about to see something that is not safe (to show or be seen) at work. In other words, if your boss found you looking at those photos when you're supposed to be working, you'd be in big trouble. Similarly, NSFL ("Not Safe For Life") is a tag that some people use to warn people that the photo they're about to see is too bloody, disgusting, or disturbing (e.g., a dead body whose head has been ripped off, a cut or wound that has become infected).