On-page SEO is when you take a look at optimizations on your actual website - content, keywords, title tags, and internal links. You are assuring the pages you create are relevant, valuable, and useful to both users and search engines.
Off-page SEO is focused on external factors such as backlinks, brand mentions, and social signals that can boost your domain authority and overall credibility.
Technical SEO mainly focuses on backend optimizations such as site speed, mobile adaptability, crawlability, structured data, and securing a connection with HTTPS.
Both on-page and off-page SEO deal with content and reputation, while technical SEO concerns whether your site can be properly accessed, indexed, and ranked by search engines.