If you know how Google Search works then it is easy to optimize according to Google. Though your site contains very good information or very good content if you couldn’t able to properly optimize to search engines it is of no use. Optimizing our page to search engines like Google is called Search Engine Optimization.
When you are searching for a particular topic, you are not searching web through Google. You are searching index of web maintained by Google. Google considers a lot of factors like how many time that key word repeat in your content ( Key word density ), what is page rank of website, is that key word in title, description and the site is quality one or not. Like this it will check many a factors.
Let us have a look at the video by Matt Cuts Engineer at quality Group Google.