The ruler measures the width of the paper from the left margin (0"). In this example, it's 7.25 inches to the right and 1.25 inches to the left of the margin, making a total page width of 8.5 inches. The width of the text area is 6 inches.
The ruler marks off the width of a page and follows the page setup information. For example, a standard letter's ruler will be a total of 8.5 inches; an A4 letter's ruler would be 21 cm (8.27 inches).
But the ruler measures from the left margin of a document, not the edge. So the scale on the ruler counts both to the left and the right, with the left margin being the start (or zero point) of the scale. See the picture on the left.
You can always move margins in the document by just sliding them along the ruler. You'll try this in the practice session.
Note You can change the measurement scale of your ruler from inches to metric on the General tab of Options dialog box (Tools menu).