The Skinny (found at the bottom of each post) is the tl;dr version of your post. You have up to 400 characters to express the core ideas of your content.
To write one, imagine you are telling someone the point of your post while escaping from a burning building, or waiting for a traffic light to turn green.
Right now, The Skinny will appear under your main header image if your content is featured on the home page. As the product develops we have some other plans for The Skinny, so it's important that you include one.