"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).
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.
At a high level, improving the "off-page SEO" of a website involves improving search engine and user perception of a site's quality. This happens by getting links from other sites, mentions of your brand, shares of your content, and "votes of confidence" from sources outside of your own website.