Screw kit, screw set or coilovers is a shock absorber and suspension combination that is adjustable. The spring is located here around the shock absorber. This ensures optimum damping without torsion loads. With some screw joints it is possible to adjust the ride height and preload with the help of a nut which rotates around the thread on the body of the shock absorber, so that the spring can be screwed up and down. More advanced adjustable screw suspension systems are fully threaded and the bottom mounting part can be changed to adjust the height of the car. They can also be equipped with a button to set the damping. The stiffness can be changed by replacing the spring for one with a different spring constant.
Many people place the screw suspension to the lowest position because it is visually preferred. However, the danger here is that the shock absorber is not calculated for this and is outside its damping range. It may even happen that the piston in the shock absorber hits the bottom of the shock absorber, causing bumpy behavior and the life of the shock absorber will be shortened. The car must be placed at the correct height or suitable springs must be placed.
There is a lot of quality difference in screw suspension. Cheaper sets often have a short life and give little to no comfort. The rule here is "cheap is often expensive to buy". Investing in a quality set offers more comfort and a longer life. Correct adjustment prolongs the life and improves the ride comfort.