In the simplest way, search engine optimization (SEO) is a technic that helps you to grow and increase traffic to your websites by high ranking in search engines.
If the website is at the top then it has more traffic and traffic leads to more benefits.
Searching is the main source to find online content when you search you find a list of websites but you open 3-4 in top among them all.
The question is how these 3 or 4 websites have reached the top? Search engine optimization is the key that will help you to rank high.