Many digital inputs in the Digital Matter product range have configurable bias resistors. The bias resistor essentially determines the state of the input when nothing is connected to the input.

There are usually 3 options:

  1. Pull up: pull the input high. 
  2. Pull down: pull the input low
  3. Disable: do not bias the input.
These options are useful when one of the states being read is open-circuit (disconnected/high impedance). In this case, enable the bias resistor setting in the opposite state to the level being switched to. 
This is commonly used when working with an "open collector" output. The pull up/down is a "weak" pull up/down.

For example: a panic button connects the input to ground when pushed, so the input needs a pull-up resistor enabled to ensure that the line is held high when the button is not pushed and is open-circuit.

The default option on DM devices is usually pull up.

Also see the note on Active Level for inputs here