SEO or Search Engine Optimisation is changing a website for the purpose of ranking higher on Search engines. In 2020 SEO is as relevant as ever and more focused on a user friendly experience.
Since the internet as we know it started, around 20 years ago, SEO has had to adapt massively. Originally people could rank a website by simply stuffing relevant keywords into the website content.
Through evolution of the search engine algorithms (that decide where a website should rank), SEO has become much more focused on a genuine user experience through relevant, well written content. Backlinks pointing to a website need to be from authoritative sources. Buying backlinks to improve a website’s ranking is no longer viable.
See this article for a more in-depth look: What is SEO?