Generally, when flaps and slats are extended, they increase the camber of the airfoil, increasing the Coefficient of Lift for the same AoA. Also, the air gaps introduced at the junctions between slats/flaps and the main wing re-energize the boundary layer on the top surface of the wing to retard flow separation on the upper surface (becomes a concern because of the increased camber).

They actually do both, depending on their deployment.