That's the idle air bleed screw. I've never had to mess with mine but you read how to properly set it here: http://www.ehow.com/i/#article_6802878
1. Loosen the jam nut on the throttle stop screw located on the passenger side of the throttle body. The throttle stop screw on the Mustang is not adjustable from the factory. If the non-adjustable screw has been replaced with an adjustable screw, it is necessary to reset the throttle stop.
2.*Insert a .010-inch feeler gauge between the throttle stop screw and the throttle lever. Tighten the screw, using a small screwdriver, until it just contacts the feeler gauge. Remove the feeler gauge, turn the screw an additional 1 1/2 turns and tighten the jam nut to hold it in place.
3.*Disconnect the idle air control valve, located on the driver-side of the throttle body, by unplugging the electrical connector. Start the engine and allow it to run for two minutes. It may be necessary to hold the throttle open slightly to keep it running during this time.
4.*Turn the idle air bleed screw, located on the throttle body near the idle air control valve, using a small screwdriver until the engine idles at 675 revolutions per minute, or RPM. Snap open the throttle body and readjust the idle air bleed screw if necessary.
5.*Turn the engine off and plug the electrical connector back into the idle air control valve. Replace the black rubber plug on the idle air bleed screw. Test drive the Mustang to verify the surging and stalling problems are fixed.