This is your piwik site id.
In the Piwik interface, when you "Add Website" you are shown a piece of JavaScript that you are told to insert into the page, in that script is a unique string that identifies the website you just defined, that is your site ID. For instance: in this code, the piwik site id is 5 ... var piwikTracker = Piwik.getTracker(pkBaseURL + "piwik.php", 5); ...