Hi,
A pull-up or pull-down resistor is basically used to preset a value (logic high or logic low) at any pin.
Find the discussion about pull-up and pull-down concept in this forum itself.
A pull-up is done by connecting a pin to VCC through a resistor (normally 1K) and a pull down is done by connnecting a pin to ground through a resistor (normally 1K). Pull-up make the preset voltage at the pin as logic high and pull-down make the preset voltage at the pin as logic low. We can change those voltages by writing a low or high respectievely.