What is the second resistor for in this potentiometer reference circuit?
It isn't "for" anything. The manufacturer is simply saying that in order to get the rated performance out of the potentiometer, whatever load is connected to the wiper should be greater than 100&times; the value of the potentiometer.

The capacitor, in conjunction with the effective source resistance of the potentiometer, creates a low-pass filter. Since the source resistance varies with the wiper position, the cutoff frequency varies accordingly. The maximum source resistance occurs at the center of the range, with a value of 1/4 the potentiometer value (assuming both ends are connected to voltage sources), and it decreases toward either end. Select the capacitor value so that the cutoff frequency stays in the range you need.