A capacitor is a device which has decreasing impedance with frequency. Thus, a series capacitor forms a high-pass filter together with the load impedance (passive guitars usually don't have series capacitors); a parallel capacitor forms a low-pass filter together with the source impedance of the pickups as higher frequencies are shunted to ground. The corner frequency of the filter is dependent upon the capacitance as well as the other impedances in the circuit; this will generally not change as the tone pot is changed, but may change according to other settings on the guitar.
In the case of a guitar, a pickup is an inductive source, so a resonant filter is formed, also known as a tuned circuit or LC circuit. This means that there will be a boost at some frequency, typically somewhere in the midrange for usual guitar combinations. Given the resistance from the volume and tone pots, this will usually only be experienced at full-on volume and full-off tone, and it is also limited by the damping effect of the pickup's resistance.
When the tone is not full on or off, we have a shelving high-cut filter. That is, the resistance of the pot acts as a limit on the HF loss--at frequencies where the capacitor is effectively zero impedance, the signal loss can be defined as a voltage divider between the source impedance and the tone pot impedance. Note that since the pickup is inductive source, if the frequency gets high enough we'd see a first-order filter, 6dB/octave drop.
In contrast, at tone full-off, we have up to a second-order high-cut (low pass) filter, where the signal loss will increase at 12dB/octave above the corner frequency (which is the -3dB point). Due to the resistances involved, typically we would have somewhere between 6dB and 12dB/octave loss at frequencies of interest.
The corner frequency, as noted above, is dependent upon the source impedance of the circuit. That can change with pickup selection and volume pot setting. Figure your neck and bridge humbuckers might be something like 8K and 11K nominal, that's not a huge difference, but a 500K volume pot set at mid-resistance (250K from wiper to each leg, this is *not* the midpoint on an audio taper pot) will have ~125K output impedance.
This becomes important where the tone pot is wired after the volume pot, which can make your tone pot darker when volume is not full up (or rather low). Some wirings have the tone pots before the volume pots, some have them after. I think typically Strats are before and LPs after. Note that if you rewire your volume pots to the "independent" configuration, you need to pay attention to the resulting effect on the tone pots, if you care.
On the other hand, in the middle pickup setting both pickups are in parallel, so their source impedance drops and corner frequency will go up very roughly by two (with volume pots full up of course, if the tone pots are after). The same effect will be created by a coil tap.
So as you can see, tone controls can be kinda crazy devices . . . I don't use them on guitar, but I use them quite a lot on bass.