A pilot hole is a smaller hole created before the required size hole is drilled, or to allow a screw to be driven in without the need for pre-drilling.
Pilot holes are beneficial because they:
Prevent Splintering and Splitting
Improve the Accuracy of the Hole Location and Execution
Make it easier to Drill a Hole, or put in a Screw or Nail, than trying to do it on an Unmarked Hard Surface