From SKYbrary Wiki
Krueger flaps are high lift devices that are fitted to all or part of the leading edge of the wings of some aircraft types. The aerodynamic effect of Krueger flaps is similar to that of slats; however, they are deployed differently. Krueger flaps are mounted on the bottom surface of the wing and are hinged at their leading edges. Actuators extend the flap down and forwards from the under surface of the wing thus increasing the wing camber which, in turn, increases lift.
Early generation jet airliners such as the B707 Series utilized Krueger flaps as their sole leading edge high lift devices. Current generation aircraft use either all slats or a combination of slats and Krueger flaps. Examples of the latter configuration include the B747 and the BOEING 737-700.