Link Juice is a term in search engine optimization that describes how much value, relevance & authority a page passes to another page because it links to it. The amount of link juice is determined by, among other things, the number of incoming and outgoing links and their quality.

Link juice somewhat refers to power of the back link

"Link Juice" is a slang term referring to how powerful a certain link is or could be. The idea is that a link from a well known website that is trusted by the search engines will pass more "juice" than one on a pointless directory.

Link Juice is the amount of positive ranking factor that a link passes from one page to the next.

This power is interpreted as a vote of recommendation toward your site and is one of the most important factors in determining your site's search ranking (and PageRank).