Switch and case?

[Astram]

What does this mean, and how do I use it. Its in the end of my staff boots script?

PHP Code:

function onTimeOut()

{

 if (this.staff_boots == "on")

  {

   for (a=0; a<4; a++)

    {

     if (keydown(a))

      {

       switch(a)

        {

          case "0":

           player.y-=this.speed;

           break;

          case "1":

           player.x-=this.speed;

           break;

          case "2":

           player.y+=this.speed;

           break;

          case "3":

           player.x+=this.speed;

           break;

        }

      }

    }

  }

 setTimer(0.05);

}