An engine block is the heavy solid-cast part of a vehicle's reciprocating engine that houses the cylinders and their components (pistons, spark plugs, etc) and is an important component for the car. They must be built strong and sturdy - commonly made of cast iron - as failure of the engine block results in failure of the entire vehicle. Since they must be built out of such a heavy, sturdy material they comprise a significant portion of the vehicle's weight.
This is where much of the waste heat form the car's internal combustion engine goes while the car is operating. This means that the block must be kept cool by use of the radiator and coolant to avoid overheating and maintain efficient operation of the vehicle.