The line-height property allows you to control the height of
each line in your text. This amount can be specified using any of the
available CSS length units: px, %, em, in, cm, mm, pt, pc, and ex. This
property is optional. If you would like to include it in your generated
CSS code simply check the checkbox by it's name and assign it a value
and a length unit. You can preview what different line heights might
look like simply by pressing the Up or Down arrows on your keyboard to
increase or decrease your line height's value. Most often, for smaller
fonts you will not need to use this property but I find if you are using
larger size fonts you may have to set this property to about 1.3em or
1.4em to allow your text to be easily legible.
Read an
official description of the line-height property from the W3C