SEO is the process of taking steps to help a website or piece of content rank higher on Google
SEO is free way to advertise. You do not pay to be the space that your advertisement is in. You are taking a piece of the online component and optimizing it so search engines like Google show it on the top of their page when someone is searching for something.
They can improve their search engine ranking by using specific words or a specific set of words.
Google rank the pages the way they do because happy customers, means more traffic, and that means more money google with receive.
The reason google does this is because they want to create a search engine that will make every single person who use it happy by getting the results they are looking for.