What is the purpose of var message?

Because as you save seen in the link I gave you, If the variable is defined in a function, it's a local variable. Now if you want to use that variable only in that function why would you create a global one although you can do that but why make things complicated.

In plain english, if you want some variable to be used in different functions, you declare that variable globally(simply less typing and more functionality), or you can actually declare the variable in different functions as well, just make sure you write "var" everytime.

Now I think writing var again and again would be a real pain in the ***, don't you think that?

Actually this stuff gets a little more complicated than what I've described but let's keep that for another day., first you got to understand this part.It's very important you understand this concept.

var number =10;  // this variable is different from the other one not the same, because you added a keyword var and created a global variable.
function num() {
var number = 5;
}

// take another eg, this time true or false

var gameStart = false;

function start() {
// some conditions here 

gameStart =true;
// what you did here is changed the global variable but this true value will be only in this function.
}

// outside this particular function gameStart is still false

Let me tell you really why are you struggling on this part because probably you din't make something on your own. Do that because that's the only way to get a clear picture really.It really helps when you create something on your own.

All being said, let's come back to the message variable, after your particular function your message variable will still be an empty string, you can change it again in another function (as message is empty).

You are not using many variables at this stage, that's why Dave basically gave you an overview how you can declare a variable and change it in function at later stage.

Actually, the reason you need to initialize the message variable to an empty string is because of the way it is used in the loop. Something like:

message += '<h2>Student: ' + student.name + '</h2>';

If you did not initialize the variable to an empty string, the value would be determined to be "undefined" initially. So your message would end up being something like "undefined<h2>Student: Luca Di Pinto</h2>". Clearly that's not what you'd want. This is just something to be aware of when concatenating strings like this.