All the measures that can be taken outside of the actual website in order to improve its position in search rankings is what referred to as "Off-Page Optimization". These are measures that help create as many high-quality backlinks (incoming links) as possible.
Many people associate off-page SEO with link building, but it goes beyond that. Many activities that don’t result in a standard link on other sites are important for off-page optimization.
On-page search engine optimization happens within the site, while off-page SEO happens outside the site. If you write a guest post for another blog or leave a comment, you’re doing off-page site promotion.