A drilled airbox is usually mated with a perfomance panel filter.
It basically involves completely removing the airbox and drilling holes on the pre-filtered side of the box to aid airflow - usually the bottom of the airbox, and then refitting the airbox.
These holes are only worth putting in a place where they cannot suck up hot engine air, on my old
Ibiza GTi I drilled the holes (10mm diameter) so that they faced the front of the car and the inner wing, no holes facing the engine. The benefits are slightly more induction noise and supposedly better air flow.