Link juice, in SEO, is a slang term that to refers the power of an internal or external link on a site. This becomes a very important factor in determining your site’s search rank and page rank.
When a well known website that is trusted by search engines links your site, it acts as a vote for your website to appear higher in the search results.
The power ie., the juice from that website is added on to your site.
While ranking, Google adds all such juices and considers this a key factor in showing your page in the search.