On-page SEO shows search crawlers what a website is about. It helps search engines see that a page is a good search result because it is relevant and useful.
Off-page SEO shows search crawlers that a website is important and valuable. It helps search engines see that a page is a good search result because the brand and website are authoritative and popular.
Let’s use Linkedin as an analogy. Think of the information on your Linkedin profile as on-page SEO. It tells other people what you’re about and what you’re good at. But the endorsements people give you, are off-page SEO. They’re votes of confidence that confirm what you’ve said about yourself is true.