Ballast :

Ballast allows the team to add weight to an already light car, with the specific intention of altering the weight distribution of the car.

Cars have a minimum weight requirement and teams need to allow for differences in the physical proportions of the driver. Ballast allows them to balance out differences and bring the car weight up to a minimum of 796kgs (including the direct, but excluding fuel). 

Photo credit 

Andrew Burden

The author Kiwi F1 Fan