Clearance Hole

A clearance hole is drilled through the top piece of a screwed joint, sized just slightly larger than the screw's shank so the threads don't bite into that piece at all. Combined with a pilot hole in the piece underneath (sized for the threads to grip), this is what lets the screw actually pull the two pieces tight together as it's driven -- without a clearance hole, the screw threads into both pieces and can hold them apart instead of clamping them.