One of the key elements of how the Google search engine works involves the use of the words, or anchor text, that appear in a link on a source page, to describe a page targeted by the link. We know this from statements about anchor text made in documents like the Lawrence Page and Sergey Brin scribed The Anatomy of a Large-Scale Hypertextual Web Search Engine and the early PageRank patents authored by Lawrence Page - Method for node ranking in a linked database and Method for scoring documents in a linked database. A newly granted patent from Google, Anchor tag indexing in a web crawler system, may provide a more detailed look at the mechanics of using anchor text as a relevancy signal for a page being linked to, by the search engine. It also describes some other processes about using links to rank pages, and crawling websites. I've written a detailed breakdown of the patent at SEO by the Sea in Google Patent on Anchor Text and Different Crawling Rates.