Keywords (or keyphrases) are what people type into search engines to find information. For instance you might type in something like "how to change a tyre" and you would hope that the results you got were the closest match possible to the info you want ie: right there on the first page of results there would be many websites to choose from who are going to tell you how to change a tyre.
When you are thinking about writing a hub you need to consider what people would type into the search engine to find the information you are offering.
You can actually look at how many searches are performed for different terms in google's keyword tool. What you want to do is find phrases that match your content with high levels of traffic but with low publisher competition.
The best place to start is to watch the hubpages learning videos on this - they show you what to do very well. http://learningcenter.hubpages.com/category/hubcam...