A safeword is primarily used to slow, pause or stop the play that you're engaging in during a scene. A safeword can be just about anything but should be something easy to remember so you don't have to think too hard when you're in Top or subspace. Typically, many kinksters use the "stoplight system" which is shown below:
Red = Hard stop
Yellow = Slow or pause
Green = Go, go, go!
If you would rather use your own safeword, you absolutely can. A personal safeword can be a word or a number and it's important to remember that it should be limited to one or two words maximum or a very short number so you can easily recall and say your safe word when you have to. The safeword is a great tool to let your play or relationship partner know that something is amiss and you need a moment or that you need to stop.
When the safeword you use may not be your own.
Your personal safeword is always yours to use except in the case of a public or private dungeon protocol. Many public or private dungeons will use a "house" safeword which is a safeword designated by the owner, manager or dungeon master (DM) for the purposes of managing a group of players within the same play space. Since the DM's job is to ensure the safety of the attendees of the dungeon as well as the property and property owner/manager, using one safeword makes it helpful to the DM to be able to instantly identify potential issues during a scene.
But what about gags?
As you may know, you can't use a safeword when you're wearing a gag, and pushing out "mmm mmm mmm" can only make your scene hotter! If you can't verbalize your safeword due to gags or other reasons, you can use something that will make a noise loud enough to catch the attention of your Top but not too loud that it startles everyone. Things like a ball or a set of keys work very well to get the attention of the person you're playing with.
In either situation, the safeword is a great tool to use to keep you safe.