This GMC Jimmy had a problem with the driver's side door. First, the door had begun to sag because the hinge pins had become badly worn.
Second, the door would swing too easily. Sometimes the door would swing shut unexpectedly (especially on a windy day or when the car was not parked on a level surface), which can cause the door to hit the person getting into the vehicle. Also, the door could open too fast and strike an adjacent car in a parking lot.
Car doors usually have two detent positions, halfway open and fully open. A detent is a device that holds something in place... for example an automatic transmission gear shift usually has detents that keep the shift linkage in the desired gear. A car door detent device may also be called a hold-open mechanism.
When a car door swings uncontrollably, it's possible that the detent mechanism has worn out.