From SKYbrary Wiki
Slats are extendable, high lift devices on the leading edge of the wings of some fixed wing aircraft. Their purpose is to increase lift during low speed operations such as takeoff, initial climb, approach and landing. They accomplish this by increasing both the surface area and the camber of the wing by deploying outwards and drooping downwards from the leading edge. In contrast, Krueger flaps increase wing camber by extending panels forward from the lower surface of the wing. Slats normally have several possible positions and extend progressively in concert with flap extension.
Slats are most often extended and retracted using hydraulically or electrically powered actuators. In some more simplistic designs, however, they are held in the retracted position by aerodynamic forces and use springs or counterweights for automatic extension at low speeds / high angles of attack.