The pinned post refers to the post pinned at the top of your social media page or profile to promote your best content. Pinning a post helps you feature an important announcement and highlights the content you want to be in the limelight. 

You can also pin posts as a follow-up to any ongoing campaign so your audience stays in a loop. Now, social media platforms also offer an option to pin comments that are great feedback or relevant to the post.

Example

The topmost post on any social media page.