It is a google currency which allows google to rank you pages on which of them is more authoritative.
it's a google buzzword.. basically means how much value an incoming link is passing to you, or you are passing out with an outbound link.
it's a term meaning how much weight a backlink carries to its destination. You need a lot of link juice to rank for a competitive term. It was coined by the seo community, not anyone at Google.
Its the secret sauce that allows to beat your competitor for ranging in Google.Link juice is far the most important aspect of dominating Google.