Resistance is the value that would be measured if using a constant current current such as method employed by a DMM set "resistance".
Impedance is an expression used that takes other things into acount that can effect the current drawn for an applied voltage no matter what the waveshape of that voltage.
For example, what is the RESISTANCE of a capacitor?
Well, if it's a good quality one, it will be infinite. At least that is what your DMM will say.
However, it's IMPEDANCE is given by the expression 1/(2.PI.f.C) where PI=3.1415926 f=frequency of voltage waveform and C is the capacitance in Farads.
(only valid for sinusoidal voltage)
Hence, if your source is a simple resistor, the impedance will be the same as the resistance. If you add a capacitor in parallel with the source, then the resistance stays the same but the impedance will decrease and will include a function similar to the equation stated above specific to any waveform (or load) applied to that source.
In the case of a PIC, the load varies as part of the action of the readadc command so the DC 'resistance' of the input also has an AC element added to it. You can reduce the source IMPEDANCE in this case by adding a capacitor to the input.
No, I don't what value because I don't know the dynamic action of the SH capacitance that causes the effect in the first place but a few 10's of nF should significantly reduce any errors caused by having too large a DC RESISTANCE. The price paid for such a reduction is reduced response time.