a safeword is a word (mainly used in BDSM - bondage, discipline,
sadism, and
masochism) that bottoms use when the painful pleasure starts to genuinely hurt, or what tops use if they think they're being too harsh.
A safeword is generally a word that isn't something you would say during sex, for example, various fruits (like apple or orange); large numbers (like
73 or
460); different shapes (like a
heptagon or a tetracontakaiheptagon); extinct animals (like a dinosaur or a
quagga);
et cetera.
A safeword is never "please stop," "I can't take it," or "it hurts," and this is because the
masochist (or bottom) says these things to express that the
sadist (or top) is doing a good job.
A safeword shouldn't be a simple word, because sometimes the word will be forgotten. A longer and more complicated word can be said, and the other will notice that it's the safeword, even if it has not been established. If you haven't set a safeword and it's too much for you, then just think of
a random word, like a phobia, or an unnecessarily long name for a color, or anything. You can even just say "safeword" and that should be enough.
Safewords should always be discussed with your other partner(s) because it's a very important thing to talk about. You can even bring it up during intercourse because it's a normal thing to ask "what's the safeword again?" and it will be fine. if you feel shy about asking, then don't be. you should be comfortable with your partner.