There are several services out there which allow you to track your website and view reports on your site visits. Many of these services require you to insert a tracking code into every page you wish to track (Google analytics for example).
The tracker code field from Web Properties | Website provides a convenient way to do this. Copy the script provided by Google analytics (or similar service) and insert it in to the tracker code field. This will cause the code to be applied to every page immediately before the closing body tag.
You can also insert tracking code for individual pages. This code will be included in addition to the code inserted in the Website tab. Simply select a page and click on the Page tab from within Web Properties and insert the code in to the Tracker code field.
The Tracker code field causes the code inserted in these fields to be included prior to the closing body tag in the exported web page. These fields are therefore a convenient way to insert other code which you want included in a particular page or every page.