It shouldn't come as a shock that I play (dom or sub) with safewords. Four of them, actually. Here's what they mean to me:
means that everything's ok. Whatever it is that's going on right now is fine. It's sort of an exception, in that it's a quick, hopefully-not-headspace-fucking response when asking if someone's ok.
Example: Someone's struggling a lot more than usual. I ask if they're still with me and they respond "green".
translates to slow down. Maybe they need a few minutes to catch their breath, maybe they're not sure if things are proceeding in a manner they're comfortable with. The scene doesn't need to stop, but it will unless something changes.
Example: I have a bad gag reflex. The dom I'm playing with shoves their dick in my mouth and I gag. "Yellow" would give me a chance to explain that, yes, I love sucking dick, but maybe ease up a little.
is stop. This is no longer ok and it is over. Untie me and let's discuss what happened if I feel like it. I might not. Doesn't matter.
Example: Tickling is a red limit for me. If I failed to explain that beforehand (it happens), I'll immediately red out of it.
Something terrible has gone wrong and this is now a matter of health and safety. End it now. Cut off the restraints if you have to.
Example: (Real world experience!) It was my first time getting flogged in a few years. We're only just getting started and WHAM. I suddenly realize that my blood sugar is essentially zilch. "Hey uh, that's black for me" and I spent an hour laying on a bed recovering.
Ok, so what about non-verbal communication? Internationally, three anything is recognized as a distress signal[Citation Needed]. Three flashes from a flashlight, a horn being blown three time, three stones in a row, etc etc. This applies when playing as well. Tap your thigh three times, grunt three times, basically do anything three times and that will mean I need to stop what I'm doing and check on you.
So, this is how I use safewords and this is the spiel I give new play partners the first time we play. This is not how everyone uses safewords. You should always discuss and agree on them with your play partners in advance of any playtime. If you want to use what I wrote above, go nuts! If you want to build on it, awesome. Treat it like a framework. If you want something simpler, there's lots of options. Just have something.