https://www.cnet.com/news/facebook-messenger-kicks-off-unsend-feature-rollout/

We already knew you'd be able to delete sent messages within 10 minutes, but TechCrunch revealed that it'd leave a marker so people would know where the message had been.

To use it, you can just tap and hold on a sent message, then hit "Remove." This gives you the option to "Remove for Everyone," completely retracting the message and leaving a marker, or "Remove for you," which lets in the recipients continue to see the message (like the current delete option).