Methods can accept properties and values that tell the method more specifically what to do, these go in the parenthesis. For instance, you might tell a child to sit down or you might want to tell them to sit down right now in the blue chair. The "right now" and "blue chair" part would be properties and values. Just like a child unfamiliar with the word might just look at you confused if you told them to sit down on the chaise, you can only tell an object or method about activities and things they are familiar with (unless you teach them about them first.) In our case, we are passing the value "Hello world!".
document.write( argument_one[, argument_two, ...])
The W3C Document Object Model (DOM)