A safe word is a word (obviously) that stops play, either for a moment or completely. This word is typically used by the submissive or bottom, but sometimes a Dom or Top will have one as well for when the person feels that the play has become too much for them to handle. This could mean that the play has reached a level they can't physically handle or maybe the play has triggered something emotionally. Often times people use different levels of safe words to distinguish the level of distress. The most frequent system used is the stop light system. Green meaning everything is fine, yellow meaning play needs to stop for a minute so they can calm down or if said during impact play it might also mean to not spank/hit so hard, and red means that play needs to stop immediately and most likely won't continue at least during that particular session. Other people use an odd word that most likely won't ever come up during a play session. This might sound like a good idea, but the problem is that if you get to a point where you're kind of out of it you might not remember the word. This is why I recommend the stop light system.
Not everybody uses safe words. Some people believe that open communication works better, while others believe that as a submissive they have no right to decide when play should stop. People that are for safe words feel that it gives them an easy way to communicate what is going on in their head. People that are against them believe that there are better ways to communicate other than using silly words.
I believe that most relationships reach a point where safe words become unnecessary, but that comes with time. I believe that people who are new to BDSM play should definitely have a safe word. If you are playing with someone that enjoys or if you yourself enjoy struggling and saying "no" during play, that makes it very difficult to distinguish between a playful no and a real no. This is where the safe word comes in. I also believe that no matter how long you've been involved in BDSM, that if you start a new relationship a safe word should be in place, even if it is never used.
Another issue that often comes up when you talk about safe words is guilt. Many submissives/bottoms feel like they have failed their Dom/Top if they safe word. I think this is absolutely ridiculous. By safe wording, you are letting your Dom/Top know what is going on in your head and you should never feel guilty about that. My Dom feels that it is my duty to use my safe word when I need to, because otherwise he might really hurt me and if that happened he could never forgive himself. And if your Dom/Top makes you feel guilty about the use of your safe word, you should seriously consider breaking off the relationship. Giving someone a safe word and then making them feel bad when they use it is counter-productive and I think games like that are ridiculous.
I hope after reading this article you feel more informed about the concept of safe words. Even if you decide they're not for you, I hope you learned something new.