Link juice is the informal, widely used term for the ranking value a hyperlink passes from the linking page to the destination. It's not evenly distributed, either: a link from a page with strong authority and few other outbound links passes more value than one from a weak page competing with fifty other links for attention.
The concept works the same way whether the link is internal or external. A strong page linking internally to a related, weaker page on the same site passes some of its authority along, which is a common, deliberate way to help newer or less-visible pages gain traction faster. A backlink from an authoritative external site works the same way in reverse, lending some of that site's credibility to whatever it links to. Nofollow, sponsored, and ugc link attributes exist specifically to withhold that transfer where a site doesn't want to vouch for a destination, which is part of why not every link on the web actually carries link juice, even though every link is still clickable.
