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.
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.
While we don't know the full algorithm Google uses to rank content, data from our Search Engine Ranking Factors study show that off-site SEO-related factors likely carry more than 50% of the ranking factor weight.
Building backlinks is at the heart of off-page SEO. Search engines use backlinks as indications of the linked-to content's quality, so a site with many high value backlinks will usually rank better than an otherwise equal site with fewer backlinks.
Regardless of how links were obtained, those that offer the greatest contribution to SEO efforts are generally those that pass the most equity. There are many signals that positively contribute to the equity passed, such as:
The linking site's popularity
How related the linking site's topic is to the site being linked to
The "freshness" of the link
The anchor text used on the linking site
The trustworthiness of the linking site
The number of other links on the linking page
Authority of the linking domain and page
While earning links from external websites is the most commonly practiced off-page SEO strategy, almost any activity that a) occurs outside of your own website and b) helps to improve your search ranking position could be thought of as "off-page SEO." These include things like:
Social media marketing
Linked and unlinked brand mentions
It's important to note, though, that the net result of each of these activities is to somehow create a reference to your site from elsewhere on the web — be that reference a link, a mention of your brand or website, or otherwise. So, the concept of truly "non-link-related" off-page SEO is actually a bit of a misnomer!