The ruler shows the width of the page in inches (other units of measurement are not supported).
Use the ruler to adjust indents (margins) for a particular paragraph. The indent markers represent the left and right indents of the paragraph containing the cursor or the first paragraph of a selection.
Click and drag the square indent marker on the left or the triangle indent marker on the right to a new position, which changes the margins for the current paragraph (the paragraph in which the blinking cursor resides) or all paragraphs in a selection.
In this graphic, the blinking cursor is in a dialogue paragraph and the indent markers have moved in the ruler bar to reflect this element's default margins.
Use the indent markers in the ruler to change the indents for a paragraph. This will override the element's defaults.
Indent Markers
There are three different indent markers available in the Ruler bar on the left:
Top Line Indent Marker
The top line indent marker, when moved to the right, will force the top line of a paragraph to indent:
Lower Lines Indent Marker
The lower lines outdent marker will indent the second through the last line in a paragraph:
Paragraph Marker
The paragraph indent marker will indent the entire a paragraph:
On the right there is only the lower lines indent marker and moving this will move the right indent for all lines in the paragraph.
Show/Hide Ruler
You can toggle the ruler on and off in the View menu (by choosing Show or Hide):
For more on when to use the margin ruler to help edit individual margins, see this article: