The door check straps is the part that stops the door opening too far and too fast.
It allows the car door to stay open fully open as well as enabling a half way stopping point when you open the door, stopping the door from swinging open too far, which can damage the hinges, while also stopping the door from slamming shut.
Positioned directly beneath the top door hinge, the door check straps connects to the door, usually with pins or bolts, and is concealed in the A-post. The components that hold the check straps to the car body are inserted through a sheet which is welded, from the inside, to the car body.
If the securing bolts or pins are loose this can damage the door strap mechanism and cause the door to rock when it is opened or closed. The metal can wear if bolts are not secured properly and this can result in needing to replace the door check straps. Parts like this do, over the course of time, wear out, corrode and stop functioning properly. When looking to replace this part with one that is compatible with your car, buying a new door check straps is the ideal choice.