When you're in the screen to edit your Hub, look to the right. The menu panel will have a listing for summary after the capsule and layout choices, right under the button to convert to video. Just click on summary and write your text.
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Georgie is correct. It is also where you can add more tags (different tab). After you write your summary, click on another tab and then click back to summary to make sure it was saved. I lost mine once and don't know why.