Resource page link building is the practice of identifying pages that curate lists of helpful external links on a given topic, and pitching your content or business for inclusion where it's genuinely relevant and useful to that page's audience.
Unlike a lot of link building, this is a naturally low-friction ask: resource pages exist specifically because the site owner wants to link out to good external content. The work is in finding pages that are a real match — not spraying pitches at every page with the word "resources" in the URL — and making a case for why your addition genuinely improves the page.