The dive brakes on this SBD Dauntless are the slotted panels visible under the wings.
Dive brakes or dive flaps are deployed to slow down an aircraft when in a dive. They often consist of a metal flap that is lowered against the air flow, thus creating drag and reducing dive speed.
In the past dive brakes were mostly used on dive bombers, which needed to dive very steeply, but not exceed their red line speed in order to drop their bombs accurately. Dive brakes are often fitted to the bottom surfaces of gliderwings as companion controls to top-surface-mounted spoilers. Most modern combat aircraft are equipped with air brakes, which perform the same function as dive brakes.