Help understand how exactly parameters work in array example.

Help understand how exactly parameters work in array example.

In the example below, the second function takes two parameters, 'elem' and index, I think I understand 'elem'.
but how does the function know that 'index' is the arrays index? same with the third example and 'array'...
are these keywords javascript recognizes? 

  

   

    // native forEach
   

  

  

   

    function printElement( elem ) {
   

  

  

   

     console.log( elem );
   

  

  

   

    }
   

  

  

   

     
   

  

  

   

    function printElementAndIndex( elem, index ) {
   

  

  

   

     console.log( "Index " + index + ": " + elem );
   

  

  

   

    }
   

  

  

   

     
   

  

  

   

    function negateElement( elem, index, array ) {
   

  

  

   

     array[ index ] = -elem;
   

  

  

   

    }