Relevance is always measured in context to a search "query string" or keywords used in a query string. The higher the relevance of a query string to a website, the higher the website would rank. Relevance of your website pertaining to query string are a function of the following:

* On-site Keyword Relevance: Keywords and query string appearing on your web pages improve your site's relevance to the same keyword phrases. This can be achieved or improved by carrying out Search Engine Optimization (SEO) on your web site using well-research, industry-relevant and important keyword phrases.
* Keywords in Anchor Text: Keywords or query string appearing in the "Anchor Text" (clickable portion) of an incoming link, irrespective of the PageRank of the linking page improve your site's relevance to those keyword phrases. Primarily based on Hilltop algorithm of Google, the Anchor Text single-handedly outweighs most associated relevance values on a linking page (although there are several more complex parameters and nuances Google studies on the linking page).
* Topic Sensitive Page Rank (TSPR): TSPR attempts to combine the value of "relevance" and "importance" of a web page, by computing the "PageRank" of your web page using only "context-sensitive" incoming links. There are no publicly available tools to measure TSPR value.