SEO or Search Engine Optimization refers to the various techniques and tactics involved in making content (websites, blogs, articles etc) rank higher in the search engine result pages (SERPs).
The person who undertakes the task of optimizing a website for search engines is also sometimes referred to as SEO or a search engine optimizer.
SEO is mainly divided into 2 parts - Onsite and Offsite optimization.
Onsite optimization means improving the structure of website in a way that could enable the search engine bots to easily crawl as well as understand the type of content the site focuses on.
Offsite optimization refers to various link building and marketing techniques that are undertaken to help the site reach a greater number of users, and at the same time helping the search engine understand the site's influence and how it is connected to other kinds of content on the web.