Uncertainty is a tolerance band around any measurement result that indicates the range of results that would be reported if the test were carried out on infinitely accurate equipment using standards held at NIST or any other internationally-recognized standards. 

Uncertainty is used to qualify measurements and their absolute accuracy and is expressed as a tolerance (Example: 422 lbf +/- 0.28 lbs -- +/- 0.28 is the uncertainty). That means that if the NIST standards were on infinitely accurate equipment, the results that would be reported would lie somewhere in the range of 400.28 lbf and 399.72 lbf.  Uncertainty estimations are developed by very detailed mathematical analysis and by observations.