Off-page SEO (also called off-site SEO) refers to actions taken outside of your own website to impact your ranking on search engine results pages (SERPs). Along with on-page SEO, these include several of the basic SEO factors that help a site rank. Off-page SEO refers to all SEO tactics that don't involve updating or publishing content to your website. Off-page SEO involves all the actions you take to impact your search engine rankings that fall outside your website.
This may include creating backlinks, being active on social media, and blogging as a guest. Off-page SEO refers to anything that is done outside of your website with the potential to affect engine rankings. Building links is the most obvious example of this, but there are many other off-page factors besides links. Off-page SEO refers to techniques you can use to improve the position of a website on the search engine results page (SERP).
Many people associate off-page SEO with link building, but it's more than that.