There are several factors that search engines use to determine what a website is about.
1.A page’s title tag.
2.A page’s Meta description.
3.A page’s Meta keywords (only very few use this any more).
4.The h1, h2,….h6 heading tags.
5.The actual content of the page.
6.Inbound anchor text.
7.Outbound anchor text.
8.Image Alt text.
9.Image Title text.
Each one of these factors combines to help tell a search engine what the web page is about. The more focused a page is on a single keyword or phrase, or a single set of semantically related keywords or phrases the better a web page will do in the SERPs. These factors combined, along with a few other factors, are known as “On-Page SEO.