Watch the video of Measurement Uncertainty
Whenever a measurement is performed, the result is never exact.
Any measurement is subject to imperfections;
some of these are due to external influences, such as short-term fluctuations in temperature, humidity, and air pressure, or variability in the performance of the measurer.
Uncertainty means the range of possible values within which the true value of the measurement lies.
Repeated measurements will show variation because of these factors
Measurement uncertainty is different from error in that it does not express a difference between two values and it does not have a sign.
Therefore it cannot be used for correcting the measurement result and cannot be regarded as an estimate of the error because the error has a sign.
Uncertainty is an unavoidable part of any measurement and it begins to matter when results are close to a specified limit.
A proper evaluation of uncertainty is a good professional practice and can provide laboratories and customers with valuable information about the quality and reliability of the result.