In a nutshell, SEO is the practice of optimising a website so that search engines will understand the relevance of your site to the search term.
The greater the relevance, the higher your site will appear in the organic (natural, unpaid) listings.
The evolution of SEO is intimidating. Sometimes even an SEO Company struggles to understand why Google made an update.
SEO is not static, and this is the one thing most people who are new to SEO understand; a proper SEO strategy will require weekly or even daily maintenance.