When a search engine like Google adds up all the "juice" from the various links it has some idea of the popularity of a website and uses it as one of the factors in ranking that website.
Thus you might often hear the term, "That won't give you any link juice". One of the misunderstandings of Google's Penguin updates (aimed at removing spammy links) is that a website with a large number of poor quality links has been penalised.
According to Google, a multitude of quality hyperlinks (or just “links”) are one of the most important factors for gaining top rankings in the Google search engine.
Link juice is the term used in the SEO world to refer to the value or equity passed from one page or site to another. This value is passed through hyperlinks. Search engines see links as votes by other websites that your page is valuable and worth promoting.