The ruler is a measurement tool found with some software programs that allow the program's user to align graphics, text, tables, or other elements on a page. When enabled the horizontal ruler is found at the top of the document and the horizontal ruler is found on the left-side of the document. In the below picture, is an example of what the ruler looks link in the Microsoft Word.
The ruler feature can be enabled or disabled in the programs View drop-down menu or through the ruler icon most commonly found above the vertical scroll bar.
Also see: Align