Please help. What does if (--) mean in javascript

 var mspf = 60;  // MS per frame
        var trailLen = 15; // Characterss per 'strand'
        var spawnTime = 20;
        var theLetters = "404 page";

        var fadeTime = mspf * trailLen * 2;  // time until trail.remove()
        function rain(){
          var randId = Math.floor(Math.random()*1000);
          myFun(trailLen,randId);


          setInterval(function(){
            $("div[id="+randId+"]").remove();
          },fadeTime);
        }

        function myFun (c,uid) {
        var id = "";
          setTimeout(function(){
            if (c == trailLen){
              var y = Math.floor(Math.random() * ($("#out").height() / 10) ) * 12; // height / 10 * 12 ensures a small space between trails, and also prevents overlapping (in theory) 
              var x = Math.floor(Math.random() * ($("#out").width() / 10) ) * 12;
              var scale = (Math.random() * 12) + 8;
              $("#out").append("<div style='font-size:"+scale+"px;top:"+y+"px;left:"+x+"px;' class='trail' id='"+uid+"'></div>"); // add .trail
            }
            if (--c){myFun(c,uid);}
            $("div[id="+uid+"]").append("<span>"+theLetters.substr(Math.floor(Math.random()*theLetters.length),1)+"</span><br>"); // adds spans to .trail 


          }, mspf);
        }

      setInterval(rain,spawnTime);
    }