What exactly are the "sealed" leading-edge slats in B737 short field performance aircraft? "The 737 design enhancements allow operators to fly increased payload in and out of airports with runways less than 5,000 feet long. The design enhancements include a two-position tail skid that enables reduced approach speeds, sealed leading-edge slats that provide increased lift during takeoff, and increased flight spoiler deflection on the ground that improves takeoff and landing performance"

Source: https://boeing.mediaroom.com/2006-07-31-Boeing-Delivers-First-737-with-Enhanced-Short-Runway-Package-to-GOL

What are the sealed leading-edge slats in B737 short field performance, and how are they different from the normal leading slats in typical B737?
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.