If you make a mistake in a video it can be difficult to correct it. If you want to re-upload the video, you need to lose all your engagement metrics. It is possible to add a note to the video's description or make a comment, but most viewers won't notice.
The new feature, called "corrections", will allow creators to add more obvious corrections. The first correction in a video can be added after it has been uploaded, but only in the top right-hand corner of the video. Clicking on the card expands the correction notes in the description.
In a video, that looks like it.
The full instructions for how to add corrections can be found here, as well as a video description of the new feature on the Creator Insider channel.
The feature is ad-hoc but still very useful. Any tool that helps creators do both the best they can and amend the record is welcome.