A digital line should not be left electrically floating when no device is actively driving it. A floating node can pick up noise and be interpreted unpredictably as either logic state.

A pull-up resistor weakly connects the line to the positive supply so the undriven state is high. A pull-down resistor similarly biases it toward ground.

A switch or open-drain output can then override that default by driving the line strongly toward the opposite rail. With a pull-up, for example, connecting the line to ground produces an active-low signal.

The resistor limits current while another device drives the opposite state. Its value must be low enough to establish a reliable logic level and acceptable rise time, but high enough to avoid unnecessary current.

Many microcontrollers provide configurable internal pull resistors, although their values are approximate and may not suit every electrical requirement.

Pull resistors turn an otherwise undefined released line into a deliberate default logic state.