Custom traffic variables, also called props (s.prop) or property variables, are counters that count the number of times each value is sent into Analytics.
Props also let you correlate custom data with specific traffic-related events. These variables are embedded in the Analytics code on each page of your website. Through s.prop variables, Analytics lets you create custom reports, unique to your organization, industry, and business objectives.
For example, if you are an automobile manufacturer, you may be interested in seeing "Most Popular Car Model" to complement your "Pages" report. You can accomplish this by allocating one of your traffic properties to represent car model. Then implement your code to pass in car model on the appropriate pages.
You can enable and name these variables in Admin Tools under Analytics > Admin > Report Suites > Edit Settings > Traffic > Traffic Variables.