Show Ruler turns the rulers in the document window on or off. The default unit for the rulers is "pixels", but by right-clicking in one of the rulers the unit can be changed.
The rulers are always showing the mouse position with a small stroke. This eases positioning the mouse.
The rulers have an additional task: they are used for creating guides. By clicking in a ruler and dragging a guide line is created that can be moved to the intended position. An already created guide can be modified and deleted with the layer tool (see chapter 6.2.1) by dragging it outside the window. Guide lines can be edited by using Setup Guides (see chapter 7.3.23), too.