It is the nut at the front of the lower control arm joint. It has a cotter pin. When it is loose, the bar slides in easier. It relieves sight misalignment. Only tighten the nut fully, I think 100 ft-lbs when wheels are on the ground and ride height adjusted. It holds the control arm bushing. If tightened, otherwise, the bushing is preload is incorrect, it will quickly fail.