When the conversation turns to someone who is not there
Speech is treated in the Qur'an as an act with weight, worth naming plainly: "do not speak ill of one another behind their backs" (49:12). The verse then asks whether you would eat the flesh of your dead brother, an image chosen for how much it repels. What we say about people when they are not in the room shapes them in the minds of everyone listening, and that is the harm being described.
Most of it does not feel like cruelty at the time. A detail gets repeated because it is interesting, then repeated again with a little more shape, and by the third telling nobody remembers who first said it or whether anyone checked. The Qur'an describes that same drift in the aftermath of a rumor that tore through a community, naming the moment people were "speaking with your mouths what you had no knowledge of" (24:15).
What the Qur'an offers in place of the habit is a standard you can apply mid-conversation. Do not pass on what you have not verified. Assume the better reading of someone's behavior when both readings fit the facts. Suspicion, once entertained, starts recruiting evidence for itself, which is why the warning falls on the listener as much as the speaker. Gossip needs an audience to survive.
Plenty of talk is worth having. Speech that calls for charity, settles a quarrel, or tells someone a hard truth they can act on is what the Qur'an protects, and the line falls around private talk doing none of those things. The measure is whether the words leave the person they concern better off, which is the question running underneath verses about honesty as well.
Being talked about is its own injury, and the Qur'an gives the wronged person room to say so aloud rather than swallow it. For anyone who has done the talking, the way back is ordinary: stop, seek forgiveness, and where you can, correct the record as publicly as you spread it. That last part is the hard half, and it is the one that repairs anything. A correction offered out loud is kindness in its most practical form.