Internal link
An internal link connects one page of your site to another page on the same domain.
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An internal link points from one page of a site to another page on the same site: menus, breadcrumbs, related-article blocks and, most usefully, links written into the body of a page. Unlike a backlink, every one of them is yours to place and to word.
What Google says about internal links
Google's link documentation is direct: "Every page you care about should have a link from at least one other page on your site." It adds that paying attention to the anchor text of internal links "can help both people and Google make sense of your site more easily and find other pages on your site" (Link best practices for Google).
On volume, Google offers no target: "There's no magical ideal number of links a given page should contain. However, if you think it's too much, then it probably is." And the link has to be crawlable, which in practice means an <a> element with an href; links that only work through script events may not be followed.
What the studies show
In Zyppy's study of 23 million internal links across 1,800 websites, URLs with 0 to 4 internal links got 2 clicks from Google Search on average, while URLs with 40 to 44 got four times as many (Zyppy). Pages with at least one exact-match anchor from an internal link had at least five times more traffic than pages without one (Zyppy).
Controlled tests point the same way. In a SearchPilot test on a site with around 8,000 regional pages, linking each location page to its nearest regions produced a 7% uplift in organic traffic; expanding a homepage footer with more internal links produced a 5% uplift to the destination pages (SearchPilot). Zyppy's figures are correlations; SearchPilot's are split tests on single sites.
Orphan and under-linked pages
A page that no other page links to is an orphan page, and it breaks Google's one rule of thumb above. Search engines may still find it through a sitemap, but it receives no internal link equity and no context from anchors. The fix is usually a few contextual links from related pages; the method is in how to find and fix orphan pages.
How to audit your internal links
Google Search Console's Links report has an internal links section that shows the pages linked most from within your own site and, for any of them, which of your pages link to it (Links report, Search Console Help). A crawler gives you the full picture, including pages with no internal links at all.
Start with the pages that matter commercially or rank just off page one, and add links to them from relevant, well-linked pages. The free internal link worklist turns a Search Console export into that list, and the internal linking strategy guide covers the rest.