What is a disable input for on a motor controller for?
This is a H-bridge motor controller which works like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Here, SW2&amp;3 are closed, so the left terminal of the motor is connected to the negative, the right to the positive terminal of the supply. You can reverse the direction by closing SW1&amp;4. (but never have 1&amp;3 or 2&amp;4 closed at the same time)

Normally, D1 is LOW and D2 is HIGH. As soon as one of the both signals is in an other state, all switches will be opened ("high impedance") and there will be no connection to any terminal of the source (Note: to switch off the motor, it's usually sufficient to open just the lower switches, or just the upper.)

If you apply a PWM signal of 70% duty cycle to D1, D1 will be HIGH 70% of the time - the switches are all open 70% of the time and closed 30%. 

As stated in the comment, this can be used for two end switches. However, the combination of one active low and one active high signal can be used for higher safety: Imagine a door which should not be opened while the motor is running. A simple door switch may fail and the controller will not notice that someone opened the door. Now you can use a change-over switch which opens one circuit and closes the other. This will not work if the switch gets stuck in one position, but it still adds some reliability to the safety system.