Probably one of the most common details of BDSM culture, safe words are words or phrases you can say to alert your partner to a problem with the scenario. Sometimes you have multiple safe words, to indicate levels of caution and comfort, sometimes you just have one to act as an on-off switch. Why do we need safe words? Because in some play, the Sub objects to the Dom's treatment and resists. Some people like this. When resisting and saying "No" becomes part of play time, it is essential to have a real No so you know when to take your partner seriously. We want to push them, but not too far.
A few things about safe words:
1. They should be easy to remember and short. It should be something you can get out in a single breath - not the Preamble to the Constitution.
2. They should be consistent. You want to have your safe word totally and completely memorized so you can remember it if you panic. Changing your safe word often is just confusing.
3. It should be something you wouldn't normally hear in playtime. For example, if your safe word is "bondage," that could very easily come up in play. My suggestion? Countries. Anyone could say "I'm serious, stop!" Very few people will think saying "France!" is sexy. I've heard that the most popular safe word is "silver" but don't quote me on that.
Note that X and I do not actually have a safe "word" for a few reasons.
1. He doesn't resist me. He bends to my will. If he ever actually resists me, I know something is wrong and I change my approach or stop. This is a huge part of our play and our trust relationship. I trust him to only resist if something is wrong and he trusts me to stop when he does.
2. 98% of the time, I gag him or muffle him, so he wouldn't be able to say a safe word anyway.
If we ever try something where he might be uncomfortable, his safe "word" is snapping his fingers. If your partner is often going to be gagged or muffled, think of behaviors they could perform. They could hum the ABCs. They could snap or beat the mattress/wall/floor three times. Whatever it is, make sure it's something clear.
Before going into a BDSM scenario with a new partner, make sure you know if they like to resist. To be honest, it would be very hard on me if X resisted. I don't like the idea of overpowering him or of forcing him into anything. That just goes against my nature. But some people like that. So make your expectations clear with your partner before playtime. And have fun!