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 appears at the top of the document, and the vertical ruler is on the left-side of the document. In the pictures below are examples of how the ruler may appear in Microsoft Word.
To change a lines indent click and hold the bottom bar (titled "Left indent" in the above picture) and drag it left to decrease the indent or right to increase the indent. If you want to adjust the hanging or first-line indent drag the up and down arrows left or right.