We have all done it, sending a text message and regretting it. A message with a wrong word in it, a text with a spelling error, or a sudden realization that you don't want to come to the party are all possibilities.
Until now, there was nothing you could do about it on iMessage except to say, "I didn't mean that!"
If you want to unsend a message, you have to do it within 15 minutes of sending it, but if you want to edit it, you have to do it within the same time frame.
If the other person is on an earlier version, it fails in an inelegant way. There may be other issues that you need to be aware of when using this feature.
It can still be useful. There is a way to use it in the newest version of the mobile operating system.
If you long-press on your message after 15 minutes, you won't be able to send or edit it.