On-site SEO → optimization is done on your website. This includes quality content, using the right keywords, always improving the loading speed of the website, and ensuring that the website is being displayed correctly on mobile phones. Your website is a moon and on-site SEO in this case is the composition of the moon and taking care of it. Off-site SEO → are travelers from other planets going to the moon. This means building quality links that lead to the moon (your website) from other websites, forums, web directories... The more visits you get - the more Google (and other search engines) will promote your website to people.