A safe word is a word used during play at any time the submissive feels uncomfortable, has reached a hard limit or needs medical attention. This could be as simple as needing a restraint adjusted or realizing they aren't enjoying the anal play as much as they’d hoped… or it could mean the scene needs to stop altogether.
This is extremely important if the game involves any play that could be harmful, but is just as important in light play as well. Even if you are only using simple restraints and a toy, a safe word is important to have! You never know when something out of the ordinary will occur and a safe word could mean the difference between a small scrape and larger gash (metaphorically speaking).
Use your safe word sparingly, slave. A safe word interrupts play and it’s not much fun if you are having to constantly stop and readjust. To avoid this kind of situation, I make sure that I am aware of your soft and hard limits. But sometimes the point of the game is to cross these limits. BDSM is a balance of trust and creating that balance can take time, so discuss your needs and desires. Like that you, slave, and me as your Mistress are on the same page.
Along with safe words, I have implemented safe signals or objects to use when you are either unable to speak or unable to move, usually during breath play or rope bondage. A safe signal could be having two fingers crossed, or the peace sign and would be used if you are gagged, wearing a hood or mask or otherwise unable to verbally communicate. A safe object is something you, slave can hold onto when tied or bound and you are unable to verbally communicate. The submissive would hold the object in his hand and drop it to signal when something is wrong.