How " list.length " provide information about how much items are inside " playList " variable?

I believe I found the part you're referencing at 5:44 in the video and he begins writing a function that looks like this:

function printList( list ) {

}

Here's what's happening. He's going to call/invoke/execute that function and when he does he's going to send it a list. Now that could be any list on the planet. It could be a playList or a todoList or a groceryList or just about anything. But whatever that list is, while this function is executing it's going to be called "list". And now we can access the length of it with list.length. You can sort of think of it as a new variable name.

So if he invokes it like this:

printList(playList);

that's going to call the printList function on the playList. We're sending the playList to the function. And when it comes into the function it gets a new name of "list". And when the function finishes executing you will no longer have access to the variable "list". This is known as "scope" and I promise you will get into more detail on that down the road. I hope that helps clear things up a bit!