Perhaps the most important off-site SEO strategy and most misunderstood strategy is something called backlinks.
Backlinks are links to your website from other websites. Google developed this part of their algorithm a long time ago and while its become more sophisticated over time, it really hasn’t changed much. The idea is that the more websites that link to your website, the more important you are. So a website with a lot of backlinks means a lot of people are talking about them for a good reason so they should rank very well. And vice versa.