The Definition page shows you information about the selected statistic. It shows the name, the title (this is what is displayed in reports), a description, default width in reports, and alignment. You can also see the Value Expression which shows the forumla using columns and variables.
Certain stats, such as this one, are unable to be edited. These stats are necessary for PT4 to calculate other stats and thus they cannot be changed. If you wish to make alterations, you can click the Duplicate button down below to create a new stat that is originally populated with the information from this stat, and then make your own changes (such as the name, width, formula, and even format type).