Google scores and indexes a webpage for keyword relevancy. The relevancy of a website's page content for Search phrases / key phrases requires on page optimization or website optimization. Google also factors a web page's content based on keyword keyword density, relevancy and search pharase proximity.
Here are a few of Google's Ranking Factors:
1. Relevancy of Title written
2. Relevancy of Meta Description approached
3. Relevancy of Body text mentioned
4. Relevant HTML tag "IMG" Alt text
5. Relevancy of HTML content
6. Page loading time
7. Number of outbound links
8. Density of keywords
9. In-linking between pages
Above are a few of the requirements for on page optimization of website's pages. Google has more than 1000's of ranking factors and some of the factors are secrets that only the People with Google know about.
