An important note can get buried fast
Posting an important correction or announcement as a comment does not help much if new comments keep pushing it down. Pinning it keeps that comment at the very top so viewers cannot miss it.
Only the channel owner can pin a comment
Pinning is an action only available to the owner of the channel that posted the video. On a video you did not upload, the option to pin simply will not appear in the three-dot menu at all.
Frequently Asked Questions
Is there a limit to how many comments I can pin?
Yes, only one comment per video can be pinned at a time. Pinning a new comment automatically unpins whichever comment was pinned before it.
Can I pin a comment someone else left?
Yes. As the channel owner, you can pin any comment on your video, not just your own, including ones left by viewers.