An internal link is any hyperlink pointing from one page on a site to another page on that same site, as opposed to a backlink coming in from somewhere else or an outbound link pointing out. They're the connective tissue of a website's structure, and they do more SEO work than their simplicity suggests.
Internal links help search engines discover new and updated pages by giving crawlers a path to follow, and they distribute ranking signal around a site: a page with strong external backlinks that links internally to a related, weaker page passes some of that authority along, which is why linking from established pages toward newer or less-visible ones is a common, effective tactic. For visitors, good internal linking surfaces related content they might not have found otherwise and keeps them engaged with the site longer. The excess the original guidance warns about is real but specific: dozens of links crammed into a page with no real relevance to each other reads as manipulative and dilutes the value each individual link carries, which is why internal links work best when every one of them genuinely helps the reader get somewhere useful.
