I have a Korg CA-30 tuner. I know they don't make them anymore, but I have a question about them. On the screen, there is the number 440. You're able to change it anywhere in the range of 400-480. It says hertz next to it, so I'm guessing it's a measurement, but I'm curious what it actually is and what's the difference between 400, 440, 480, or any number in between.

it should be at 440 for guitar.
I wondered why the frisbee was getting bigger, then it hit me.
The frequency of the note A. It's usually at 440Hz, but in rare situation it'll be a bit off (say, when you're playing with an orchestra and they tune to 443Hz), so you can adjust it on chromatic tuners.