Instagram said a couple of months ago that it was testing the ability to pin comments to the top of your posts, and it looks like that test went well because the feature is now rolling out widely.
Starting today, Instagram is pushing out the ability to pin comments to everyone. This feature allows you to pin up to 3 comments at the top of your post.
This feature is meant to help highlight a positive or insightful comment on your post and foster good conversations.
To pin a comment, just swipe on it and your should see a pin icon next to the reply, report, and delete buttons. When you pin a comment, the person who made it will be notified.