“Off-page SEO” which is sometimes called “off-site SEO” refers to a set of actions actions taken outside of your website to impact your rankings within search engine results pages.
Optimizing for off-site SEO involves improving search engine and user perception of a site’s popularity, relevance, trustworthiness, and authority. Sites with a high Domain Authority will have a greater impact.
To do this connect to reputable places on the Internet (pages, sites, people, etc.) linking to or promoting your website, and in doing so they are effectively “vouching” for the quality of your content.