Link juice is the SEO value and authority that flows from one page to another through a hyperlink. It is a foundational concept of Google's PageRank algorithm and is also called "link equity."
The mechanism is straightforward. When a high-authority page (say, a major news site) links to your page, a portion of its authority "flows" to yours. The more of this inflow you receive, the more likely your page is to rank higher in search results.
Link juice is distributed roughly equally. If a page contains 10 outbound links, each destination receives about one-tenth of that page's authority. Consequently, a link from a page with few outbound links is worth more per link.
The relationship between short URLs and link juice is an important topic for SEO practitioners. A 301 redirect (permanent) passes link juice almost entirely, and Google states that redirects do not cause a loss of PageRank. The difference between 301 and 302 is not how much link juice they pass but which URL search engines treat as canonical: a 301 makes the destination the canonical URL, whereas with a 302 the source URL tends to remain in search results.
The rel="nofollow" attribute is a tool for controlling link juice. Adding nofollow to a link blocks the transfer of juice. It is standard practice to apply nofollow to advertising links and links within user-generated content. In 2019, Google also introduced rel="sponsored" (for ads) and rel="ugc" (for user-generated content), enabling finer-grained link classification.