Calibration uncertainty is a property of a measurement result that defines the range of probable values of the measurand.
Uncertainty means the amount of “doubt” in the calibration process, so it tells how “good” the calibration process was. Uncertainty can be caused by various sources, such as the device under test, the reference standard, calibration method or environmental conditions.
In the worst case, if the uncertainty of the calibration process is larger than the accuracy or tolerance level of the device under calibration, then calibration does not make much sense.
See more at What is calibration