a conductor string string connect with a vechile which have any explosive material and it regularly in contect with ground explain it

When the vehicle moves, its body surface experiences air friction. This produces some amount of charge on the body. When such small charges accommodate in  large quantity, it can take the form of spark which can lead to heavy damage due to explosive material inside. So to avoid such damage, a conducting string attached to body of the vehicle is always on Contact with the ground so that the small charges produced on the surface move to earth (earthing) providing no harm to the vehicle and the material inside it.