What does the ‘Move To Top’ button do?

The 'Move to Top' button in your Queued Posts lets you move a post to the top of your list, for that particular social account.

Let's suppose you have scheduled 4 posts for your Twitter profile and 4 posts for your Facebook Page. Now, you want to move the Facebook Page's post to the top. So, clicking on the 'Move to  Top' button will move this post to the top of your scheduled post list for Facebook Page (not to the top of your queue).