Start by thinking about what the capacitor will be doing to different frequencies on your signal line.
Recall that the impedance of a capacitor is Zc = 1/(jωC); as frequency increases (ω = 2*pi*freq), ω gets larger, and Zc gets smaller. A capacitor will act like an "AC short" at higher frequencies. If the capacitor is connected from your signal line to ground, then higher frequencies will get "short circuited" to ground. If you have a short circuit at a particular frequency, then it won't go into the following stage.
Using that basic concept, you can see that a shunt RC is a simple low-pass filter; it will help keep high frequency signals out of your ADC's input. Usually high frequency content will appear as noise and spikes/spurs in your low frequency desired signal, so filtering them off is usually desired.