What is this aluminium thing under my car

That is a skid plate. A skid plate is a protective plate made of metal or composite material that is attached to the underside of a vehicle, such as a car or off-road vehicle, to protect the undercarriage from damage caused by rough or uneven terrain. Skid plates are often used in vehicles designed for off-road use or in areas where the roads are poorly maintained. They are also used to protect the undercarriage of a vehicle from damage caused by rocks, debris, or even animals that may be on or near the road.