Original: When an outflow boundary forms due to a shallow layer of rain-cooled air spreading out near ground level from the parent thunderstorm, both speed and directional wind shear can result at the leading edge of the three dimensional boundary.
Simplified: When an outflow boundary moves away from a thunderstorm due to a shallow layer of rain-cooled air that spreads out at ground level, both speed and directional wind shear can result at the leading edge of the three dimensional boundary.