Hi! This a quite wide question, however, I'll try to explain shortly.
On-page SEO looks at what your site (or your page) is about
Off-page SEO looks at how authoritative and popular your site is
Put simply, what you rank for is largely determined by on-page factors, while how high you rank in the search results is largely determined by off-page factors.
The biggest off-page SEO factor is the number and quality of backlinks to your website. Some examples of ways you can build links to your website are:
Creating awesome content that people want to link to because it is valuable.
Social media shares of your content that ultimately generate links. (if you have an ahrefs subcription, check backlink profile of this casino website https://vulkanplatinum.com.ru/ to understand better about SMM)
Outreach e-mails to influencers in your industry that ultimately link to you.
Guest blogging on sites related to yours.
These guest posts will have links back to your site.