The chassis is the frame which the body and engine are mounted to. The chassis is the most important of the three main elements of the vehicle when it comes to handling.
The shape of the frame structure and how it connects to the suspension components. The design can't be entirely focused on handling performance, however, because the other components in the vehicle must be considered in the design.
The frame shape is designed in a manner similar to architecture, because it must be built to either withstand forces or channel them to another location (and some components must be made moderately weak so they absorb collision impacts. A rigid frame directs forces more efficently, making a vehicle handle more predictably. A lot of rigidity in cars comes from the roof and window pillars, and because of this, convertible cars require extra reinforcement at the bottom to make up for the lost rigidity.
The chassis structure is designed to channel forces across itself evenly, without sudden changes, and retaining its shape as much as possible.
Just like how a building needs the right kind of structural support to keep from falling down under its own weight, vehicles need architecture to withstand acelleration, braking, cornering, and even collision forces.