Timing for a typical ADC in a microcontroller, when you need accurate results, can be a fine balancing act.
There are two basic properties to consider - the sample, or acquisition, time, and the conversion time.
Crudely put, an ADC can be seen as a capacitor which gets switched either to be charged from the analog input pin, or have its voltage read by the sampling system. This is known as Sample And Hold.
During the sample time the capacitor is connected to the analog input pin. During this time it charges up to the level of the incoming voltage. That charging up isn't instantaneous, but is still very fast. You have to ensure that the capacitor is connected long enough for the voltage across the capacitor to match, as closely as possible, the incoming voltage.
Once you have captured that voltage it's time to convert it into a digital value. The most common way (and the way the PIC18 uses) is a Successive Approximation ADC. That takes, over a number of clock periods, an ever more high resolution look at the voltage - typically one clock per bit of output data, so 12 clock cycles for a 12-bit ADC.
Each of the 12 bits of sample take a certain amount of time to calculate, so you have to go slow enough for that conversion to complete properly. But, at the same time, the capacitor is discharging. Go too slow, and your sample will lose accuracy. So you have to make sure that your timing is fast enough to get good accurate results, but not so fast the ADC can't sample the values properly.