Go to "My Account" from the right hand top of the page and select "edit" on your Hub (one Hub at a time).
On the right hand side of the page you will see 3 boxes.
Go to the 3rd box that has "reorder, tags, settings, SUMMARY, and group".
Click on "SUMMARY" and then you can type a unique summary of what your article is about.
When you are done, be sure to click "Done Editing" at the top of the page.
Any Hub that has an "A" after it in your statistics/my account page, means that you need to add a summary. You will need to repeat these steps for each Hub that you have published.
I hope this helps!