why we are using headerdiv?

I guess it's a div :

var HeaderDivObj= document.getElementById('#yourdivid');

And then you are trying to create a table inside this div :

var HeadertableObj = HeaderDivObj.appendChild(document.createElement('table'));

Now What is appendchild?
The appendChild() method appends a node as the last child of a node.

Refer : http://www.w3schools.com/jsref/met_node_appendchild.asp[^]

Good luck