The value selects the source of the ADC conversion clock. For a detailed answer to this, look at "Selecting the ADC Conversion Clock" sub-section, in the ADC section of your chosen PICs Datasheet. Some newer PICs have a wider selection range than provided by the DEFINE, in those cases it's best to select by presetting the Registers yourself. If you are unsure what source to use, the RC option (which is an internal ADC option and nothing whatsoever to do with the PICs main Oscillator) is always a safe bet.