Have you ever tried to explain Metrology to your family? I have taken my Dad into my lab showed him exactly what I do and he still thinks all I do is work on computers and fix VCR's. When I was in the Air Force he kept asking me when I was going to fly my plane home.
When people ask me what I do for a living, I use the example of a factory. I tell them anything that has to be accurate when they design or manufacture it, we are the people who make sure that happens. For example, when they make engine parts for an automobile, they use instruments to make sure those parts are the right size. When you tune your television to channel 3, they use instruments to make sure it is accurately on channel 3. When you listen to the radio they use instruments to make sure the station is operating on the frequency it is supposed to be on. We are the ones who make sure all of those instruments read correctly, and read the same. That includes length, weight, pressure, flow, temperature, humidity, voltage, current, light intensity, frequency, and a whole lot more.
When I explain it that way, most people get it. Don't know if that helps. My older son used to always think that "I fixed computers." But I like the "factory" illustration since most people understand that.
I always tell people " Have you ever heard of someone challenging a speeding ticket based on thier speedometer not reading correctly? I am the guy at the shop that would verify if it was reading correctly or not" Seems to be the briefest explanation possible with a concept most people can relate to.
The easiest way I have found is explaining the gas pump or the scale at the grocery store.