In SEO, some of the efforts and techniques are applied to the website that you are working on for optimization. All those techniques come under on-page SEO since you are putting effort into your own website. These include quality content, use of keywords in content, improving the loading speed of the site, using an SSL certificate, meta tags, etc.
On the other hand, off-page SEO includes efforts and techniques on third-party websites to create links to your site. For example, you will create links to your own site from high-authority websites like Forbes, Times of India, etc. This tells Google that some authority websites are referring to you. It eventually helps in improved rankings.