Interval cardio consist of bouts of higher intensity exertion followed by a recovery period, then repeated for several rounds. For example: after 5 minutes of warm up on an air dyne stationary bike you would do a high intensity bout (7-10 perceived exertion) for 10-30 seconds followed by a recovery bout for about 30-60 seconds. This would be 1 interval. Repeat for 5-10 intervals.