SEO refers to techniques used to gather more hits from search engines. These techniques include smart titling and keyword usage; you want to build your title and your content so that when someone performs a related search, your hub will show up. For example, if you were writing a hub about making money on HubPages, your title might be "Can I make money on HubPages?" and your keywords would be things like "make money on HubPages" "make money online" "make money writing," and so on.
It's not too hard to do and it can increase your traffic too.