could anyone break down each line of this code?

Ok, I am going to walk you through the code and then answer your questions below. Before that however, I recommend you go back and re-watch the videos covering functions in JavaScript, as all the sections after are going to assume that you understand how they work.

Now onto the code:

var playList = [
 [ 'i did it my way', 'frank sinatra'],
 [ 'respect', 'aretha franklin'],
 [ 'imagine', 'john lennon'],
 [ 'some song', 'someone']
];

This creates an array the contains arrays as its elements. In programming we call an array that contains arrays, a multi-dimensional array. These are widely used in programming specifically in programs that require vectors, or in machine learning. I recommend you make sure you understand how they work.

Basically playlist is an array that looks like this:

As you can see you can match the row and column #s to find a specific value: (0, 0) = "I did it my way" for example. Using the same logic we can get any value in a multi-dimensional array by doing the following: playlist[0][0] which would give us the value "I did it my way".

function print(message) {
  document.write(message);
}

This function takes a parameter called message, and then writes whatever message is to the browser using the document.write() method.

function printSongs(songs) {
  var listHTML = '<ol>';
  for ( var i = 0; i < songs.length; i += 1) {
    listHTML += '<li>' + songs[i][0] + ' by ' + songs[i][1] + '</li>';
  }
  listHTML += '</ol>';
  print(listHTML);
}

This is the big one. printSongs(songs) is a function that takes a parameter called songs. In the first line of the function we establish a string called listHTML it currently holds the string that represents the ordered list tag for html. We then create a loop that loops as many times as their are elements in the song parameter (meaning that song is either a string or some sort of list collection). We then concatenate listHTML with another string that contains a list item tag, along with two specific variable values: songs[i][0] and songs[i][1]. Since we are treating song as though it is a multi-dimensional array then we can safely assume that printSongs() accepts a multi-dimensional array as a parameter.

Now from the first part of the code we discussed, we know that we are basically printing all of the rows and columns of the multi-dimensional array.

Next we concatenate the closing ordered list tag to the listHTML string, and then using the print() method created earlier we print it to the html document the script is attached to. Because of the nature of document.write, and the fact that we used html tags, multi-dimensional array will be printed as an ordered list to the html page.

printSongs(playList);

This line simply calls the printSongs() method and then gives it the parameter of the multi-dimensional array we created earlier.

Now onto your specific questions:

On the 'function print(message)' and 'function printSongs(songs)', we named the parameters to 'message' and 'songs' as placeholders. Is it correct?

Yes, parameters are basically placeholders for actual variable values, so the parameter for print() is message and the parameter for printSongs() is songs. Remember that a parameter can be anything that we put in the method that is valid javascript. So we could have written printSongs(38); but of course that would lead to errors because a number is not what the code is expecting so when it reaches the loop within printSongs, it will freak and error out.

We didn't declare/define 'songs' parameter on any of the code before the 'function printSongs(songs) ' line. How can we make a for loop with the parameter 'songs'?

We don't have to as it is a parameter, remember that javascript is a dynamically typed language, meaning that we don't have to specify the type of the variable that we create, the same applies for parameters. Based on how we use the parameters within the function, will give javascript's compiler and idea of what the variable we expect is. Remember, that a parameters is just a placeholder at the end of the day. If we were to define or declare song in any way, we would change the value of the variable that we place into the function.

For example:

var num = 10;
function printNum(n) {
    console.log(n);
}
printNum(num);  // ====> Prints 10 to the console

var num = 10;
function printNum(n) {
    n = 18;
    console.log(n);
}
printNum(num);  // ====> Prints 18 to the console

Do you get it now? Hopefully that helps, and again, I recommend you rewatch the video and also the videos on functions.