Calibration is a process of computing accurate camera positions and orientations from a video of user waving a small glowing object called marker (for color/color+depth cameras). This step is essential and required for multi-camera system setup.
Once you calibrated the camera system, you should not move your cameras for subsequent video shoots. If you move at least one camera, you need to perform calibration again.
We recommend to run calibration twice - before and after capture session. If any camera was moved during capture session, calibration made after the session can give you correct camera positions.