One of my site with a pagerank "6".. but what does it mean? How I know the pagerank is referring to what keyword? any way to check?
Page Rank has no relationship with Keywords. "not clear, that's why post here.." You want to find more information about Page Rank? If yes, then you can read this google.com/corporate/tech.html
It 'was' a measure of the credibility of your website. However, now with recent updates, its unlikely that you can actually use PageRank as a measure of anything, its a nice to have but dont use it as a sign of keyword rankings becuase they are not related. Its just nice 'eye candy' if you have high PR. Just focus on keyword optimisation and rankings not PR and you shoudl see some good results.
If you set up a google webmaster tools account you'll be able to see the keywords are used in external links to your site. They're listed in the "What Googlebot Sees" section.