What is the off-page SEO?
"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.
Why does off-page SEO matter?
While search algorithms and ranking factors are constantly changing, the general consensus within the SEO community is that the relevance, trustworthiness, and authority that effective off-page SEO affords a website still play a major role in a page's ability to rank.