You can "fix" it or not--your choice. Having a summary for each Hub is optional.
If you choose to fix it, you first open a Hub into Edit mode. Then on the right side, two to the right of the "tags" button is a button for "summary". Click on that, write yourself a summary, save, and you're done.
I personally do NOT like the concept--because (a) Google (and the other search engines) seem to grab text lines FOR me that are better than most of those I've tried writing myself and (b) I find writing summaries to be a royal pain, very difficult and usually not very good.
But that's just me. It's your option.