cross browser matchesSelector

Question

I'm very new to cross-browser web development. Not quiet understand this:

    (function(doc) {
     var matches; 
     matches = doc.matchesSelector || 
     doc.webkitMatchesSelector || 
     doc.mozMatchesSelector || 
     doc.oMatchesSelector || 
     doc.msMatchesSelector;
    })(document.documentElement);

   window.addEventListener('click', function(e) { 
       if ( matches.call(e.target, 'ul a') ) {
       alert('Event delegation and matchesSelector are BFFs'); 
       e.preventDefault();}
   },false);

So what does document.documentElement.matchesSelector return? What's the meaning of matches=doc.matchesSelector if every vendor prefixes are included after?