I'm not sure if I like Artie's analogy for caps.
How about this: the cap determines the cut-off point where the frequency band is "treble" or "not treble". Everything above that point will be dumped to ground as the tone knob is turned down.
If you use a higher value cap (i.e. .047), the "treble" zone will reach futher down, so when you turn the tone knob down, you will be removing more mids and producing mud. If you use a higher value cap (i.e. .01), the "treble" zone starts higher, so if you turn down the tone knob, you'll be removing just the high frequencies and leaving the mids (and maybe high-mids) alone.
So...the pot opens and closes the valve, and the cap determines what frequency range that valve covers.