When your page (or any page) is shown up in Google (or any other search engine), below the heading there is some description (of 1-2 lines). That is the summary of your hub.
1. If you do not write your summary (you can write it in hub edit tool), the opening lines will become your description/summary in the google's pages.
2. If you write the summary/description, those specific lines will become the description in the search engine pages. If you write it good, it can induce more people to click and bring more traffic.
I hope my answered help.