Social bookmarking and it's importance

what is social bookmarking and it’s importance in SEO?

These are also a kind of Bookmarking sites but they are high in Page Rank. Like normal bookmarking sites is only 1,2 or maximum 3 PR but social bookmarking sites are around 1-8 PR. Also it can easily help to rank your website. Cos Google gives preferences to high PR back links.