Safewords are widely used throughout the BDSM community as one of our main safety precautions when playing with people. A safeword is a random and non-relevant word used to either stop or pause a scene. The safeword needs to be something out of the ordinary so that it isn't mixed up in the heat of the moment or during consensual non-consent play. Words like stop, no, please and don't can all be used within scenes without actually intending them to stop a scene. Therefore the safeword needs to be something totally non-related to the play.
People using a single safeword tend to use it as a stop or pause when a limit has been reached. Some people use what is known as the traffic light system; green, yellow and red. This is more used as a way to discreetly check in with one another without really disrupting the mood of the scene. The phrase "what's your colour?" (of similar variations) is often used to do this.
Green means good, the scene is going well, no limits have been neared and the scene can continue on as planned. A response of yellow is a sign to slow down or take a moment to pause a re-evaluate. This usually means that you are approaching a limit a need to proceed with a little extra caution in order to continue with the scene and not exceed the previously agreed upon limits. Red is a definitive stop. It means that all play stops, bonds are released, the scene is over. At this point, you have either reached or surpassed a limit.
For casual play and when playing with new people, I'd suggest the traffic light system as it is a nice easy way to check in throughout play. A single general use safeword works for most regular play partners and relationships once you know each other. By this point you know each other's limits and body language, so you typically don't need to check in.
For some in relationships, like myself and Sir, the safeword doesn't necessarily stop play. I have the utmost trust in Sir and for this reason, I have very few limits. When I use my safeword, it's more to let Sir know that I don't think I can take much more of whatever we are doing at the time. This results in Sir taking a moment to reassess, sometimes adjust what we are doing, talk to me and assure me that I'm okay and then continue on with our play. However, this usually means an in-depth discussion afterwards about why the safeword was used, how I felt and a little extra aftercare taken as well.
A safeword can be as simple as you like, ours is red. Or as random as possible, something like pineapple! But at the end of the day, I believe that everyone involved in BDSM should have a safeword.