My answer here is only my opinion, so I'm not sure (even after a few years on HubPages) exactly what HubPages' definition of "unique" would be; but I think "unique" requires one or more of the following things:
1. A Hub shouldn't look like every other web article on any given thing. For example, several months ago I searched online for something on hives. I kept running into articles that "all looked alike". They were generally 400- or 500-word articles that gave a nutshell version of what hives looks like and what most often causes it. I got aggravated as I kept coming up with the same nutshell version; because it meant I had to work a little harder to find material that was more comprehensive. What made it particularly aggravating was that I kept coming up with quickie articles that told me what I pretty much already knew myself (and what most other adults usually know as well).
In that case, something unique would have been particularly comprehensive or may have offered something from an angle not usually offered by the "average web article", written according to the usual "rules" for web articles. Your Hub shouldn't look like yet another one of those same articles on the same thing.
2. Another way to make a Hub unique (and this is an important one) is to add capsules and links that make it a "center of information" /resources related to the subject.
3. Your material shouldn't be posted (at least in the same words) as it is elsewhere online. You can write an article about shoes on another site, and then write another article about shoes on HubPages; but if you post the same article on both sites (or many sites) your Hub/article won't be unique because it appears in other places online besides HubPages.