i understand the function of a pull up or pull down resistor, pull up increases the voltage at a node and a pull down decreases the voltage
1. If a pull-up resistor is being used and the pin it’s pulling on is grounded, the pin becomes a direct path to ground (a short for the power-supply). The resistor prevents it from being a direct short. Same thing, applies for a pull-down resistor, although slightly less direct. And,
2. The resistor is necessary to create ‘potential’. On one side of the resistor is the power-source (either ground or positive). On the other side is the pin and whatever current potential is on it. If the two are different, they will try to equalize by flowing through the resistor towards the power-supply (effectively pulling the signal in that direction).
Hope that helps….