On-Page SEO means improving things inside your website to help it rank better. This includes good content, proper titles, headings, keywords, fast loading, mobile-friendly design, and internal links.
Off-Page SEO means improving your website’s rank by doing things outside your website. This includes getting backlinks from other sites, sharing on social media, and building your website’s reputation.
In short:
On-Page SEO = work done on your website.
Off-Page SEO = work done outside your website.