What do the dots mean in relative file path?

I am new to programming and learning php on my own. I have two question about the following relative path of a file

$fp = fopen ("$_SERVER[DOCUMENT_ROOT]/../orders/orders.txt", 'w');

My questions about the relative path in the code above are that:

1. What I understand, $_SERVER[DOCUMENT_ROOT] points to the very root directory of the file structure like htdocs, www or public_html on different servers. Please guide if I am understanding it correctly?
2. What do the two dots mean in the path above?

Thank You

Answer

.. means the parent directory, so it goes one level up there and into a sibling directory of your document root called orders.