"Off-page SEO" (also called "off-site SEO") refers to actions taken outside of your own website to impact your rankings within search engine results pages (SERPs).
Optimizing for off-site ranking factors involves improving search engine and user perception of a site's popularity, relevance, trustworthiness, and authority. This is accomplished by other reputable places on the Internet (pages, sites, people, etc.) linking to or promoting your website, and effectively "vouching" for the quality of your content.
