Social bookmarking works in the same way as you would bookmark a favorite webpage to your browser except that there are sites specifically designated for bookmarking whatever you want. That could be a new blog post you make, an article or a video you want people to see.
Bookmarking sites get a lot of traffic, so it's especially good for your blog posts. There are many good bookmarking sites: DIGG, Stumble Upon, Propeller, Sphinn, Tumblr, Newsvine, Mixx and a whole slew of others.
But rather than manually submit your info to each one of these which can take a long time, try to use a social bookmarking submitter. OnlyWire.com is a good one, both free and paid service. Yes, you do have to create an account with each site you want to bookmark with, but OnlyWire saves your passwords for each. After that, it's just click and send. Your stuff is posted all at once.
Try not to bookmark more than ten sites per day so you don't get accused of spamming.