Search Engine Optimisation is the practice of making a website more optimised for search engines. The goal is to get higher on the search engines for a particular keyword in an attempt to gain more traffic, or clicks onto the website.
There are a multitude of ways to keep your site best optimised. On-page SEO is what makes your page and its content optimisable for the crawlers that read your web pages. They will determine if your content is relevant to the keywords and that will contribute towards your ranking. Then you have off page SEO which pertains to backlinks. Backlinks are links on other websites that link back to your website. If a high authority website mentions yours with a link, this will give you a better citation flow, or “link juice”.