That ruler is for controlling paragraph level text formatting in a rich text document. You can show/hide it through the menu:
- WriteRoom > Format > Text > Show Formatting Toolbar
WriteRoom doesn't include docs on it, but if you want the details open TextEdit.app (comes with all Macs) and look in the "Format documents" section of the help book.
With all of that said, I would recommend against use it in the first place. Just hide it, when you need to really format your document you are better off using an app like Pages.app that's really designed for formatting. For WriteRoom I recommend that you just use plaintext documents, or if you use rich text just use basic bold, italic, style formatting.