Search engines use anchor text to determine what the hyperlinked page is about. Anchor text should reflect the content of the page the hyperlink is pointing to. When optimizing a page for search engines, including important keywords in the anchor text can make a difference in the ranking of your page.
Search engines have different metrics for determining search engine rankings, and link relevancy is one of them. The source page's content and the content of the anchor text helps determine link relevancy.
You can track anchor text by using search engine optimization (SEO) tools like Google Webmaster Tools. Google's Webmaster Tools allows webmasters to see the most common words in anchor text linking to their site.
Learn more about anchor text:
Learn about optimizing anchor text and SEO Moz.
Read about how anchor text is an important search ranking factor.