Sticky Posts are posts that will stick to the top of your front page.
The video starts with using sticky posts in the TwentyFourteen theme and then shows the simpler method used in other themes.
They are particularly powerful when using the TwentyFourteen where the feature images of a sticky posts can be displayed in a grid across the top of the site.
In other themes the posts will just sit at the top of the new posts and not move down when new content is added.
How to make Posts Sticky.
The easiest way is in the Publish Metabox, you edit the visibility of the post and tick the box for sticky.
Using the TwentyFourteen theme you can stick posts with a particular tag to the top of your blog instead. To control that you use the Customizer.
In this screenshot we are using the tag ‘featured’ to stick to the top and we are hiding the tag.