Bookmarking sites are site that you 'virtually' bookmark sites you like, so you type in the URL of a site you like and people can see that you have bookmarked it and they may click that link and visit the site. If you bookmark your own sites/blogs/hubs people may like the look of the link and click it.
This will give you traffic to your blog/site/hub and therefore increase your traffic. My advice would be to find one bookmarking site and set your sights on it. Concentrate on one site and make a few friends, that's the only way you'll get 'targeted traffic' for me I only use 'Digg' and 'StumbleUpon' I focus all my energy on them so I get the best results.
Also they can be used for backlinking purposes which will increase your Google/other search engine's rank and you'll get more traffic (which will then again increase your earnings).
Sorry if that's a bit complicated, over time you'll learn a lot about SEO, and traffic so don't worry if it seems a bit overwhelming at first.