Every internal link passes a fraction of the source page's authority to the target — a mechanism inherited from the original PageRank model and still load-bearing in modern search. Pages with many high-quality internal links rank better; pages with none (orphans) often do not rank at all.
The most-neglected pattern is contextual links inside body copy. Sidebar links and footer links count, but contextual links from relevant paragraphs carry more weight because they signal topical relationship.
Anchor text matters. Descriptive anchors ('how to set up double opt-in') outperform generic anchors ('click here') for both user experience and ranking. Just do not over-optimise — exact-match anchors used at scale look manipulative.