Source Page Rank
The rank of an external source page can pass SEO strength to a destination page on your website though a link, subject to the link not being marked with the nofollow attribute.
This means that when search engines are crawling a page on another site and they discover a link to your site, they may attribute some of the rank value of the source page to your page.
This is the main principle of link building – to gain links that pass rank value, often called ‘link juice’ from the source site to the destination page.
Google’s John Mueller confirmed some time ago that Googlebot doesn’t actually pass through the link at time of crawling, but it does take a not of the link, and if not expressly disallowed will schedule a visit to the destination page.
This impacts the ‘crawl rate’ of the destination site and acts as a positive ‘vote’ toward the site. Both factors are important for gaining rank.