You can give it to each youser that you want or mark by this vaiable any action, and then you will hava ability to find data by this marker. This marker need to be used in cases, where GA-team didn't create metric. Custom variable make GA more plastic. But persanally I never used it. More about custom variable here - code.google.com/intl/en-En/apis/analytics/docs/tracking/gaTrackingCustomVariables.html