Downforce is created when air moves through and over parts of the car.
A formula cars wings are set at angles which force the air up and through them,
for every force there is an opposite force
therefore as the air is forced upwards it also creates a force pushing downwards.
This is achieved without making the car heavier than it already is.
Downforce pushes the car into the road harder increasing the friction between the tyres and the road allowing the car take corners at higher speeds and the driver to have more control.