it is the primary structure of the wing, bounded by the spars, which carries the airloads into the fuselage (or the fuselage loads to the air). in very light planes the wing structure can be a single spar with a strcutural D-nose ahead of it.
Just a few points to add to what Rb1957 has already mentioned.
The wingbox includes the structural members of the wing such as spars, ribs, braces, bars, struts etc. and is enclosed by the upper and lower skins of the wing. Generally the roots of the front and the rear spars are connected to the fuselage. The spars typically resist the bending and twisting loads and the shearing loads are taken on by the skin.
Modern day, large transport category aircrafts have their fuel tanks inside the wings. Hence the term 'wet wings'.
