MatchScore is a measure of how well a page matches a given set of search words.
MatchScore measures how closely search words entered match a given web page. I don't know if Google uses that exact term, or has another term to explain the same thing. I just use the word "MatchScore" to help me explain the search process. You heard it here first!
MatchScore is calculated at the time the searching person is looking for something. (Unlike PageRank, which is calculated after Google scans a web page). PageRank is independent of search words and is calculated per page. MatchScore is calculated per page, per search. You can't see, feel or smell MatchScore. You only know that a page has gotten a relatively high one when it appears in first place in search results!
The user enters "life coaching dublin" and the web page:
http://www.dublin.ie/health/life-coaching.htm
scores a very high MatchScore because the words are in its URL (the web address itself), in its Title Tag (displays at the top of browser when you view the page) and throughout the page's content. (The link, below, explains how MatchScore is calculated).
For argument's sake, the MatchScore might be A- for the search words "life coaching dublin".
If the user had typed "life coaching stockholm", the same web page might only score a C+.
If the user had entered "plastic fishing nets", that same web page might score an F (for Fail). No doubt, some other web page would have gotten a high MatchScore.
Search engines work out how closely each of the billion+ pages on the web match the search words entered by the searching person, and produce MatchScore. Of course, search engines have many fancy ways of not having to check every darn page on the 'net every time a search is requested, but the effect is the same; the better the search engine matches the web page, the more valuable the search engine is to its customers.
Search engines aren't in the business of dark magic. Their interests are well served when people find exactly what they are looking for.
Help search engines match your web page with search words by doing all the right things, described at the link below, that make your web page get a high MatchScore when people go looking for what you do.