How to pull a pin low? On the instructions for this motor driver, it says that motors can be put to sleep by "pulling a pin low".


  SLP - Logic Input. When pulled LOW, outputs are disabled and power consumption is minimized.  


I'm guess this means connect to ground? How could I trigger that via my Arduino board?
Connect it to an IO port and set it to OUTPUT and LOW. That's what it means to "pull low".

It helps if you understand how an IO port works. This may help you.