What is important to know about calibration?

Calibration is the process we use to ensure the measurement instruments are accurate as per their specifications,
Imagine the clock you depend on each day would be wrong, or the altimeter in an airplane indicates that you fly 200 meter higher than in reality,
What if the cable tester you used to certify a building gave a “pass”, but it actually failed?
How to recognize 3rd party calibrations?
How should a credible calibration report look like?
What is the concept of Calibration?