This is easy. When in 'Edit' mode up top click on the second tab that says "Summary Text". Uncheck the box that says 'Calculate summary automatically'
Then in the 'Summary' box below type in your short description. HubPages takes what you type in here and uses this as the web pages 'Meta Description' within the hub's HTML code automatically.
Title also becomes the Title in the HTML code too.
You have no way to to add Meta Keywords, but those are pretty much worthless anyways, so no worries there.