NIST or National Institute of Standards and Technology is now part of the U.S. Department of Commerce and is one of the nation’s oldest physical science laboratories. Follow this link for more info: https://www.nist.gov/about-nist

In a nutshell, they are the top level to which all measurements referenced to NIST are traced back to. Another link to a different section of their website explaining Traceability: https://www.nist.gov/traceability/traceability-table-contents

Yes. Calibrations are performed using standards that have an unbroken chain traceable to NIST, and all calibrations have been accredited by an outside body to ISO/IEC 17025.

If you need the uncertainty aspect of a measurement (usually because you in turn calibrate equipment or equipment for your customers or your quality system has this requirement) then you will need an Accredited Calibration. This type of calibration includes the uncertainty measurement of the calibration process.

You can learn more about that here.