When you tune, you are either tuning to a note from another musician–we usually tune to the box player, if there is one, as he can’t change his pitch and we can change ours–or, to get a starting point, you can pick a good note (like first octave G xxx|ooo ) and tune it against a digital tuner until it centers the needle (or lights the green light). Note the italics: when you tune with a tuner, all it gives you is a starting point–you will have to listen and adjust as you go. Fact is, you will need to always do that, no matter how you tune.
Some basics:
Ok, a musical note has a pitch, which is a measure of how “high” or “low” it is. Each note on a whistle is a different pitch. An accurate way of measuring pitch is in vibrations per second, or Hertz. Most sessions tune so that A is 440 vibrations per second, or A-440. You can look at this tone on an oscilloscope and it’ll look a lot like a sine wave.
If you have two musicians, both playing whistle, and they both play A, and are exactly in tune at A-440, they will sound almost like one whistle. However, if one is two low (“flat,” or not quite enough vibrations per second) or too high (“sharp,” or a few too many vibrations per second), then the sine waves that they are producing don’t line up exactly any more. As the peaks of the waves come closer together, match, and then move apart again, it produces an interference pattern–an audible “beat.” If they are almost in tune, then the beats come very slowly. As the pitches get further apart, the beats come faster and faster.
When you play a note and someone else plays the same note, you’ll hear the beats. You need to adjust in the direction that makes the beats slower, not faster–do this often enough, and you’ll get the hang of hearing if you’re too high or too low–and when the beats stop, then you are in tune.
Note that humans being, well, human, it’s pretty rare to ever get the beats to completely stop. The idea is to get as close as you can, and then keep listening as you play, and keep adjusting, forever honing in on the elusive (and sometimes moving!) target.
I hope this helps.
–James