Link juice refers to the value that a web page gets when it's linked to by a website that has lots of visitors and a good reputation.
Search engines judge the value of a web page in part by the number of links to it from other websites (rather than from other pages on the same site). That's because links from other sites - called inbound links - tell the engines that people find the information on the web page valuable - otherwise, they wouldn't bother linking to it.
Link juice refers to the benefit that a web page gets when another web page links to it. Link juice is passed if the site carrying the link has high authority. The page that the high-authority site links to must also be relevant to both the anchor text on which the link is built and the content of the page that hosts the link.
For more information about the value of inbound links to a website, read: