All the techniques and efforts you apply to your website, be it publishing blogs or writing website content, are called on-page SEO. On-page SEO includes keyword research to write quality content for the website, including those keywords in blogs and web content, work on meta tags, enhance the loading speed of the site, etc.
Off-page SEO is when your efforts and techniques are directed toward third-party websites to build powerful links to your website. It includes creating links from a high-authority website to your site, writing guest posts, etc. Off-page SEO strategies tell Google that authority and trusted websites refer to your site, improving your search engine ranking.