Fuel tank foam that you fill the petrol tank with to prevent weight shifting and petrol sloshing.
With foam in the tank, you improve the overall appearance of the kart. In a normal kart, you have about 6-7 kilos of petrol that is thrown around in the curves, the weight in the tank can be perceived as significantly more as the g-forces in the curves throw 6-7 kilos of petrol sideways in each curve.
The same goes for acceleration and braking, with foam in the tank you almost eliminate loose weight that is thrown back and forth.
The cubes are 5x5 cm and when you fill the petrol tank with them, it only takes up 1-2% of the space so the tank then holds basically as much as before.
The foam is made of special material that makes the life of the foam over 10 years.