A "resistive input" is obviously an input with a relatively high impedance, such as a voltage divider. The risk is that the ADC input may load this, and alter the intended voltage - and it may do so in a manner that is not constant over time, but rather pulse-like.
Many microcontroller ADC inputs have an effective input impedance which is actually a function of the software-configured sample and/or clock rate. Other factors such as internal gain may also play a role. Generally, the faster you sample, the lower impedance the input is going to present (ie, the more it will load the driving circuit).
Some recent microcontrollers have an op-amp buffer stage you can enable in front of the ADC. If not, you can build your own op-amp voltage follower externally.