A lowering block is a small rectangular slab of aluminum that is used to adjust the suspension height of a vehicle that uses leaf springs.
A lowering block is placed between the springs and the rear axle. This brings the axle closer to the frame of the vehicle, effectively lowering the rear end. Some types of trucks have a rear axle that rides above the leaf spring. Installing lowering blocks on a truck like this will actually raise the rear of the truck.
The blocks are commonly available at most auto parts stores in sizes of 1" 2" and 3" at prices ranging from $10 to $30 USD. (I am not sure if metric versions exist in other countries, but it is a possibility). 4" blocks also exist, but most stores do not stock them, but you can simply place 2 of the 2" blocks together for the same effect. It is not reccomended to use more than 4" of lowering blocks on a vehicle. Nor should you install them on the front, (for those rare older cars that have front leaf springs).
In todays world lowering blocks are most often used in combination with a torsion bar adjustment. To provide a cheap lowering job an a compact truck.