Off-page SEO is how Google measures your domain's authority and relevance beyond your own website. The main signals are backlinks (hyperlinks from other domains pointing to yours), brand mentions (your company name cited without a link), and digital PR (earned media and press coverage). Google treats these signals as votes of confidence — if other websites link to or mention you, Google infers your content is trustworthy.
The key difference between off-page and on-page: you don't directly control off-page signals. You can't force another website to link to you. Instead, you earn these signals by creating valuable content, building relationships, and generating buzz. This is why off-page SEO takes longer than on-page work. You're dependent on other people's decisions.
For a practical view: if on-page SEO is optimizing what you control (title tags, content quality, structure), off-page SEO is creating the conditions where other people want to link to you or mention you. Both are necessary. Neither alone moves the needle.