On
aircraft, a 'slat' is a motorized extension that pops out of the
leading edge of the
wing. Slats are used along with
flaps to increase the
camber of the wing, thus generating more
lift at the expense of aerodynamics.
Slats only appear on widebody aircraft: smaller planes don't have them. Those of you who have read Michael Crichton's Airframe will know that deploying the slats at cruising speed causes the plane to gyrate wildly and throw everyone around the cabin until one of the geniuses in the cockpit pulls them back in. This is why they are only used on takeoff or landing, when the plane is moving more slowly.