Screw Pilot Hole
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A pilot hole removes wood from the path of the screw's threads, preventing the wood from splitting as the screw forces its way through, especially near the end of a board or in dense or brittle species. Without one, driving a screw can split thin stock outright even at low torque.

The pilot hole diameter should roughly match the screw's minor (root) diameter (the shaft diameter without the threads), not the major diameter over the threads. This lets the threads bite into surrounding wood while removing enough material that the screw doesn't have to displace it by force.

When joining two boards, the top board needs a larger clearance hole matching the screw's major diameter so the screw can spin freely and pull the boards together; only the bottom board needs the smaller pilot hole so the threads can grip.

Yes. Hardwoods and dense or brittle species need a pilot hole closer to the screw's major diameter to avoid splitting, while soft, forgiving species like pine can often use a slightly smaller pilot hole since the wood compresses around the threads instead of cracking.

Countersinking is a separate step from pilot drilling, cutting a conical recess so a flat-head screw sits flush with or below the surface; you typically drill the pilot hole and countersink either as two operations or with a combination bit that does both in one pass.