The summary on HubPages is known as the description tag for SEO purposes. Most often, the description tag you write is the one that will be shown when searchers are looking for your topic on Google. For instance, this is my summary on a Hub and this is exactly what Google chose to display for the search query:
WHAT IS SEO? by Writer Fox
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What is SEO? Find definition & full info on SEO – Search Engine Optimization. How to do it yourself, SEO videos, keyword research, SEO content, links & more.
The real purpose of the description tag is to encourage people to click through to your site. There is more information about how to write a description tag in my HubPage article above.