The Screwdriver appears to be a slotted or "flat-blade" screwdriver with a blue hexagon handle
The Screwdriver is required to unlock a little compartment located on the sides of the stairs in the Secret Area, close to the Basement tunnel. An item useful for the player's escape will always be found inside.
It spawns on a shelf near the ceiling of the second floor, just above the stairs and to the right of the Main Door. To knock it down, you need either a Crossbow and one of the Tranquilizer Darts, obtainable with the Weapon Key, or the Shotgun and ammo, obtainable by bringing all three shotgun pieces down into the Garage.
The screwdriver has no other usage, but it is still made important because it's one of the hardest items to obtain.