When a Moderator comments on an Idea, the new entry is typically displayed in-line with other comments. However, if you click Pin this comment on top of the thread the comment will be displayed above all the existing comments regardless of the comment sorting order.
 
The Moderator is the only class of users able to use this function. However, a pinned comment will be visible to all members of the community.

Moderator's pinned comment is displayed above all other comments as shown in screenshot below with a pin icon and name of the person who pinned it.

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