What exactly are the "sealed" leading-edge slats in B737 short field performance aircraft?
A "sealed" slat has no slot between it and the main wing.  According to b737.org.uk, sealing flaps allows for more lift at a lower AOA (Angle of Attack).

Reference from Flightglobal.com "Short Field 737 goes into flight test" states that sealing of up to 25 degrees of slats will be available for take-off, an improvement from 1 and 5 degrees.  Slots (not sealed) are to be used for landings.

This information  indicates "sealing" creates a large single undercambered surface, which would have a lift to drag (less drag) profile more suited for take-off and climb out than the slotted settings.