The rotation matrix together with the translation vector allows you to transform points from the world coordinate to the camera coordinate system. Please go through the following documentation to learn how extrinsics are calculated.
The extrinsics function computes the rotation matrix and translation vector for a single image in closed form. During calibration, the extrinsics are estimated numerically to minimize the reprojection errors for all calibration images.
The following documentation might also be useful if you want estimate the accuracy of camera parameters estimated using cameraCalibrator app.