Social bookmarking is a way of categorising or 'tagging' a web site or web page, so you can save it for later. Most browsers allow you to bookmark pages so you can return to preferred web content. Social bookmarks achieve the same end, but by storing the information on the internet it is much easier to share with colleagues and friends. You can also access your preferences from any computer with internet access.
Centralising people's preferred content in this way, allows the social bookmark sites to promote pages that are popular with more people.
Pages can be 'tagged' with a free set of category labels, which allows natural grouping of content to emerge. This arguably leads to more intuitive division of content and more relevant search results.
If you find a particular blog interesting you can save it for future reference by simply clicking on one of the social bookmark links at the bottom of each article. The link will take to that social bookmarking site where you will have to register for full access to services. The sites are free once you are registered.
Wikipedia on 'Social Bookmarking'
Read about social bookmarking on Wikipedia