Link Juice is an SEO slang term; a way to conceptualize the good effect created by links obtained to a page from other websites. A link from a highly ranked, popular website to a page of one's own website creates a lot of "link juice". A link from a poorly ranked, unpopular website creates very little "link juice". One can envision the "power" of these links as a kind of juice poured into one's website pages. Routing that link juice to other pages of one's website without wasting any of it is an artform.
