3D cut is a special additional cut. Inserted into the leading edge of the wing, it results in a cleaner, more aerodynamic shape to the leading edge, in the inflated shape during flight.
Paragliders are made by multiple parts which are cut as 2 dimensional flat pieces of fabric.
Imagine trying to fold a straight long piece of paper into the shape of a panel, specifically in the leading edge part. You will discover that in order to receive the curvature needed on the center of the “panel” excess material will be left on the edges which are typically sewn to the ribs.
A 3D cut allows to trim out this excess material, resulting in a cleaner, less wrinkled profile.
In other words the connected panel through a 3D cut can no longer lay flat as a 2D piece of fabric, but is perfect as a curved 3 dimensional shape!