The greater the space, the better the rigidity.
A lightweight core of polymeric foam, to cite one example, can be sandwiched between two skins of fiber composite, sheet metal, or film. The core lends the skins their shape, spacing them apart from each other evenly. Because of the space between the skins, the core significantly increases the rigidity of the composite.
The weight of the core material is, however, significantly lower than that of the additional skins that would be necessary to achieve comparable rigidity in the absence of a core. The core material must nevertheless be able to withstand high stresses. That is, all impact must be transmitted from one skin to the other and the compressive forces fully absorbed.