From Wikipedia:

A torsion box consists of two skins applied to a core material, usually a grid or framework of some kind. The torsion box functions as a beam, but is considerably lighter than a solid beam of the same size without losing much strength. Torsion boxes are used in the construction of airframes, especially wings and vertical stabilizers, in making wooden tables and doors, and for skis and snowboards.

Here Beth gives an assist with strength testing and some much needed compression.

The assembled grid-core.