On-page SEO refers to SEO factors and techniques focused on optimizing the aspects of your website that are under your control. Off-page SEO refers to SEO factors and strategies focused on promoting your site or brand on the web. On-page SEO involves all the on-site techniques you can employ to ensure that a web page is ranked in a SERP. It can also help determine how well that page ranks.
On-page SEO is about optimizing the web pages of your website. In on-page SEO, your strategy is very much under your control. Off-page SEO, on the other hand, is about improving your SEO through other means, such as creating content and getting backlinks from other websites. These tactics give credibility to your website.
If you want to use the best SEO practices, on-page and off-page SEO are great starting points.