"Off-page SEO" (also referred to "off-site SEO") refers to efforts and actions executed outside of your business' website to positively impact on your search engine rankings (SERPs).
According to MOZ
Optimising 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.
Establishing trust within the search engines is very important and one way for Google to ascertain "proof" that your business / brand is what you say and claim it to be is from off-site factors such as social media, trusted websites and review platforms. In the old days one deceptive tactic black hat SEO technique to dupe the likes of Google into thinking your site was popular and worth ranking was link building.
Wordpress define link building as: "The process of getting other websites to link back to your website. All marketers and business owners should be interested in building links to drive referral traffic and increase their site's authority."
Link building should come with the caveat of to exercise with caution. One strong authoritative link in your industry is worth more than 1000 crummy links. If anything lots of poor quality links can harm your website's search engine optimisation effectiveness and performance. If something sounds too good to be true, with promises of lots of links and quick rankings, the chances will be that it is too good to be true.
Also, there is an argument you don't even need to have a link due to the concepts of co-occurence and co-citation
However, to harness the power of co-citation and co-occurence is only possible when the trustworthiness of your brand and content of your website is good enough to be shared and mentioned with others. With this in mind it leaves no question why Google puts so much emphasis on shareability (social media sharing) and social media optimisation.