Ok, let's see if I can remember what was taught in all those electronics lectures I slept through... Basically, impedance can be thought of as resistance of sorts.. A kind of obstruction to the flow of electricity through the circuit... Another thing to keep in mind is that each note has its own frequency.. For example, an A4 has a frequency of 440Hz.. The formulas that Keith posted will give you the impedance for a certain frequency... The first formula, in particular, shows that as the frequency of the note gets higher (higher in pitch), the more resistance the capacitor is going to put up when faced with a flow of electricity..
In English, it blocks the lower pitched sounds more than the high pitched ones... (Isn't school just so full of crap??)
A tone circuit looks something like the one below.. The zig-zagged lines represent the tone and volume pots, which are actually variable resistors...
Let's say the pot is 250k.. This is actually the max resistance the pot can show.. Let's say as well that the capacitor is a 0.022 uF one... Let's also take 2 notes, an A6 (1760Hz) and an A1 (55.0Hz)... Using Keith's formulas, you'll get an impedance of 4.110k for the A6 and 131.53k for the lower A1... As you can see, the A6 is gonna get a lot less resistance to it's flow...
Bearing this in mind, let's look at the circuit.. At position 1 (solid arrow), the tone is cranked to the MAX... What happens is that the impedance from the capacitor is added to the 250k resistance which the pot is now showing.. When this happens, both the A6 and the A1 now have a RELATIVELY similar amount of resistance to their flows.. What this means is that much of the signal for both notes will go into the volume pot and out to your amp...
When the tone is all the way MIN at 2 (dashed arrow), the tone pot is actually taken out of the circuit (so to speak).. This means that the difference in resistance seen by the A6 and the A1 are now RELATIVELY different, with the A1 seeing much more resistance.. What then happens is that the A6 would go into ground (and into oblivion) while the A1 would go to the volume pot and out to your amp, resulting in a muddier, bassier sound...
One last thing to note (in relation to your question, Rhoads) is that with any tone circuit, there is always going to be a little bit of signal running off into the ground, even if your tone pot is maxed out.. What this means is that if you use a 0.047uF capacitor with a 250k pot, you will get less treble even when your tone is up all the way.. You can get a clearer sound by using either a smaller cap (0.022uF) or a bigger pot (500k).. This is again that relative difference thing..
I hope this is clear.. Quite frankly, it's not the easiest thing in the world to understand.. I think I'll go play my guitar for a while.. With the tone up, of course..