Hard limits are absolute no-gos; soft limits are things someone is wary of but might try under the right conditions.
The distinction is simple and the whole point of it is that it's boring to say out loud before anything happens. A hard limit is a wall: an act you will not do, full stop, no persuasion, no "just this once." A soft limit is a door with conditions on it: something you might be curious about but only with the right person, the right trust, the right amount of warning. The work isn't in having limits — everyone has them — it's in naming them out loud to a partner before the lights go down.
The common misconception is that limits are the killjoy part, the paperwork you get through so the fun can start. In practice the opposite is true: knowing exactly where the wall is is what makes the rest of the room safe to move around in. Limits are also not carved in stone. A hard limit can soften with trust and experience; a soft limit can harden after a bad night. That's not inconsistency, it's the reason the conversation has to happen every time and not just once.
Where it goes wrong is the soft limit. A soft limit is a "yes, if" — not an open invitation, and definitely not a "no" that a top gets to talk someone out of. "Pushing limits" is a real thing people do, but it's negotiated in advance and out loud, never decided unilaterally mid-scene by the person holding the flogger. Crossing a hard limit — or a soft one without explicit permission — is how the community identifies someone as unsafe, not adventurous.
If you've only ever seen these words on a kink checklist (the tick-box lists where you rate acts from "hard limit" to "enthusiastic yes"), that's the domesticated version. The list is a conversation starter, not the conversation.