Calibration is making sure your lens is oriented around the correct view.  This must be done before you start shooting your video, as you won't be able to recalibrate afterward!

This is what a poorly calibrated film can look like:





Before you begin filming your video, make sure to center the green donut around your lens view, as shown below.  You can pinch your fingers to make the donut bigger or smaller, accordingly, and then hug it around your lens view.  

Poor Calibration:                                                    Good Calibration:
(notice the silver area around your view)