Link juice
Link juice is SEO slang for the ranking value a link passes from one page to another. It means the same as link equity.
Link juice is SEO slang for the ranking value one page passes to another when it links to it. It means the same as link equity, which is the term you'll see in more formal writing. The picture is of value flowing through links like liquid through pipes: a strong page has plenty to pass on, and each link carries some of it away.
You'll hear it in phrases like "that link passes a lot of juice" or "nofollow links don't pass juice."
How people use the idea
The metaphor is handy for two everyday decisions. Off your site, it explains why one link from a respected page in your field can be worth more than dozens from weak ones. On your site, it's a reminder to link from your strongest pages, usually the homepage and your most-linked guides, to the pages you most want to rank.
For example, Kestrel Roasters' brewing guide has links from 60 other sites. Adding a link from it to Kestrel's new filter coffee page passes some of that strength along, and helps the new page get found and ranked sooner.
Where the metaphor stops working
It makes linking sound like plumbing, with a fixed amount of juice to split up. Search engines are less mechanical than that, and Google weighs links alongside many other signals. It's best used as a rough rule of thumb.
It also says nothing about AI answers. A link may pass juice, but an AI platform reads the text of a page. What gets you named in an answer is what the page says about you, which is why a mention without a link can matter as much there.