If they are referring to back link juice to get more Google PR, then that is what they are talking about, I think.
It's if your website have a backlink from any website/page with a PR and it's not a 'NoFollow' link, then it will pass some PR value to your website
Slang just like Google Juice in referring how to gain traffic. Matt Cutt's first used the term I believed.
link juice is the juice given by website dofollow to your website. It helps you to increase your website PR
This time of year if you can get a good glass of eggnog with your link juice, it certainly adds more weight as well!! </sarcasm>