So here is the scenario. You show up to play with rest of the group and on the first note everyone stops playing. They turn and look at you with disdain. Your instrument is out of tune! Well, that never happens. Okay, well there was that one time when I took that workshop. But the point being, one cannot play with others if their instrument is not in tune. Specifically, that means not just being in tune with itself but also in tune with everyone else.
Now some persons are gifted with "perfect pitch". This means they can hear a note and tell you what it is. A very small percentage of people can actually do this naturally, but for the majority it is a learned behavior. For all the rest of us enter the "electronic tuner"! There are many different kinds.
Guess what! They all work. That's right. I mention this because they are frequently marketed as being for a specific instrument. This is only sort of correct. It's not about the instrument, it's the range. For instance a D4 is a D4. It does not matter if it is on a Mountain Dulcimer or a Ukulele. D4 is the fourth D by octave up from the lowest octave on the piano keyboard.
But here is the good part: a D4 is 293.66 hz on any instrument or from any source. An E4 is 329.63hz. An F4 is 349.23hz and so on. There's a chart; Google it. You can find the frequency of any note by sharp, flat, or natural from the lowest to the highest.
Bear with me. It really isn't that complicated. The number 293.66 is the numerical value of D4, for instance. It is the amount of vibration of sound per second for that particular note. Vibration of what? Air. Sound is vibration. If something vibrates and causes air to move (as in our case, strings) at a specific frequency (as in our example, 293.66 hz) The result is D4. The oscillating or vibrating air waves hit our ear drums and our brain comes to recognize that frequency as D4.
Cool huh? So what is that "hz" thing? It stands for "hertz". Heinrich Rudolf Hertz, was the first person to provide conclusive proof of the existence of electromagnetic waves. It measures frequency of electromagnetic waves in "cycles" per second.
Way back in the day when I was learning electronics (shortly after the earth's crust cooled when dinosaurs roamed the earth, actually late 1960's) We still called it "cycles". The "hz" thing didn't start until the mid-60's. Hertz means cycles. Originally it was used to describe cycles of electrical energy. Now it is used to describe the cycle of anything. In our case it is sound waves. (Sort of a footnote to history: The whole electromagnetic thing didn't pay Heinrich too well so he had to come up with a day job. He opened a used car lot which also did not work too well. So he started to rent out his cars instead. Thus started the "Hertz Rental Car" chain. No, that's not true.)
Getting on with one of the "great secrets of the universe"; electronic tuners come in several varieties. There is the ever popular "Snark" which works great until you lose it. This is true for any brand of clip-on type tuner. Other types simply set on your instrument while you tune it.
Finally, another type of tuner is becoming more popular. Since one usually sets aside a consider percentage of their brain cells for the task of not losing their cell phone; Smartphone tuning apps are becoming a popular choice. There are dozens from which to choose.
"Gstring" is one of the most popular, available in Android as well as Apple. It's also free.
In a smartphone app the sound is "heard" by the phone microphone which then converts it to electromagnetic energy (Hertz again).
This is "analogous" to the sound wave. This analogue signal is then digitized so that the phone's computer can then show it on the phone's screen so that we can know if our instrument is in tune.
It is so easy to keep one's instrument in tune. So do it! Frequently. It couldn't hertz.