Link juice is nothing but the power of the backlink you have from other sites. If you have backlink from high Google Page Ranking site, then you more link juice. And remember link juice has a great power when it comes to Search Engine Optimization.
Link juice is inbound links, which you get from other Websites. Link juice is an important aspect of SEO, if you have inbound links from Website's with PR>=2, better is the lnk juice, and more are the chance to Rank better in SERP.
To increase your juice you need to get more links, especially from sites that have a lot of juice.Improving rankings is all about a) targeting the right keywords, and b) getting the right kinds of links.
The SEO term 'link juice' refers to the search engine 'trust' i.e. credibility, and/or page rank conveyed by a link from one site to another. A site with high 'trust' or page rank provides more link juice than one that doesn't.