A slat is a thin airfoil deployed from the leading edge of the wing. This acts as a new little wing, but its objective is not to produce lift but to generate the circulation needed for it. Slat circulation will be opposite to wing circulation reducing the highest speed of the boundary layer. This reduces the maximum lift also, making its distribution along the wing softer, but allowing the boundary layer to detach later (by reducing the adverse pressure generated in the trailing edge).
Usually, slats are used with flaps during take off and landing operations as both produce extra lift at low speed.