The ruler tool lets you quick draw a ruler to demonstrate measurement.
The ruler is slightly offset from the line that defines it. This is so that you can use the ruler to measure a line without the ruler covering up what you are trying to measure. The amount of offset can be changed by right clicking on the ruler and editing its properties.
Height is the height of the ruler. You can set the height of the ruler as low as zero and just use the scale.
Margin controls how much space there is between the edge of the ruler and the beginning of the scale.
Offset controls the amount the ruler is offset from the line it is measuring.
Reversing the scale lets you draw "left-handed" rulers. Starting the scale at a number apart from 0 allows you to construct number line like rulers.
FX Draw will produce rulers marked in mm, cm, inches and points. The rulers will be true to scale unless you change the scale factor. The scale factor lets you create rulers that are not true to scale if you need to.