Internal links are links between pages on your own site, like a blog post linking to a product page or another article.
Why it matters
They help visitors find related information, help search engines discover and understand your pages, and pass authority from your strongest pages to others. Pages that nothing links to ("orphan pages") are easy for both people and search engines to miss.
Example
A guide to brewing methods that links to your pour-over kit and to your grind-size article sends readers deeper into your site and tells Google those pages matter.
What to do
Link new articles to relevant existing pages, and go back to add links from older articles to new ones. Check regularly for broken internal links and orphan pages.