The chassis is the underlying frame that holds a vehicle together — the engine, transmission, axles, cab, and cargo area are all attached to it. Think of it as the skeleton that everything else is built on.
The word comes up often in intermodal shipping too, where a “chassis” is the wheeled frame a shipping container is set onto so a truck can haul it. In both senses, it’s the base structure that makes the load roadworthy.