"Why are safe words important?"
This is a real question, submitted by self-identified "dominant." I use the lower case d because any Dominant worth the name knows the answer to this question.
Let me emphasize how VITALLY IMPORTANT safe words, and their clear presentation, are to submissives and Dominants during any type of play. Safe words provide the assurance of safety, sanity, and consent.
A safe word is the signal that can be given by the submissive at any point during play that causes ALL activity to stop immediately. No matter when it is uttered, the Dominant must stop what is happening and check in with the submissive to find out whether the scene needs to end, be modified, or if the sub needs to have a break.
I prefer my subs to use the word RED. Simple, effective, not easily mistaken for another word, and not a word commonly uttered by itself. I also instruct my subs to use the word YELLOW to signal to me when they want a certain activity scaled back, but not stopped entirely. In play during which the mouth is gagged, I implement the three tap signal. Three taps, and play stops.
Am important note: while the Dominant must instruct the sub in use of safe words, it is the responsibility of BOTH parties to communicate and ensure safety throughout the session. The sub must be willing and able to communicate his or her comfort or discomfort.