The cross member is a structural part of the car body which holds the engine and/or transmission in place.
The cross member is more often than not made of steel, and usually contained within a box that is bolted across the underside of the car, to the bottom of the chassis.
The cross member forms part of the bodywork of the car and therefore is built to be strong and solid and able to withstand the weight of the engine and/or transmission. Being a structural part, the cross member assists with the overall handling of the car and helps to keep the body panels solidly in place and in the correct position.
It is vital that the cross member is in sound condition and working properly as it is paramount to the structural safety of the car. Prone to rust caused by water from the road and damaged caused by debris from the road, over time the cross member will eventually need replacing. If damaged the cross member will need replacing with one that is compatible with the make and model of your vehicle.