They refer to the [multiple] mechanical joints between each wing and the central "wing box" inside the fuselage (also called "wing torsion box"), which absorbs the torsion stresses imparted by each wing along the flight (and which also usually stows the undercarriage and -at least in big, multi-engined jets- is also the main fuel tank.

(Ja veus: De Manresa a Terrassa passant per Syracuse, NY... De fet, millor que Terrassa - Sabadell, oi?)