It is literally the science of measurement, it has practical products that are often used for instrument calibration. Metrology proper is an academic field much like any other hard science, and just as chemists created the foundation on which chemical engineers and swimming pool boys perform their jobs, metrologists create new methods of measurement and the foundation for routine calibration labs.
In other words, they seek to answer the question, "How do you measure the trueness of the world's most accurate[correct] or most precise[resolution] tool?"
Both relative [comparative, is A or B better?] and for a known uncertainty on a fixed scale. [Z odds of it being within X deviation of Y]
Uncertainty grows as you use one instrument to calibrate another and that instrument is used to calibrate yet another... in a chain away from a fixed standard. As each calibration has a range [say +-1 unit] the exact position within said range is unknown [+0.006 or -0.9?] and those unknowns are tracked and combined and traceable [as is any good scientific data] This is the mundane accounting portion.
The real action involves the theory behind unit definitions (ie, exactly what is temperature measuring, should degrees be a linear portion of that thing? Can a time standard be defined using only the other base measurements or must we rely on a natural physical element?) and in developing better standards (replace the king's foot with a new platinum-alloy bar kept in a climate controlled room, and that in turn was replaced by the wavelength of a specific frequency of light in a vacuum)