The HyperCube can be considered the heart of most visualizations. It's the cube definition provided to the Qlik Data Engine, which holds all information on which data is queried and how it is calculated. If selections are applied to a HyperCube, only the selected values are displayed.
A HyperCube can be visually imagined like a reqular Straight Table in Qlik, with a collection of dimensions and metrics as well as additional attributes.
As it's effectively describing how all data is processed, the qHyperCubeDef object plays a significant role in building visualizations or other components that heavily rely on the Qlik engine for calculation.