Hypervigilance is a symptom of the hyperarousal you get when you have PTSD (post traumatic stress disorder). After you are exposed to a traumatic, life-threatening event and develop PTSD, your fear circuits in your brain centered on the amygdala become activated. This in turn activates the fight or flight response, and your arousal and stress levels increase to the point where your body is on alert and constantly scanning your environment for any signs of danger. So you are easily startled, and jumpy, where any sudden noise will activate your fear circuits and fight or flight response to peak levels.
So hypervigilance, which is the constant scanning around you for any signs of danger, is a symptom of hyperarousal, which is the increased stress response that occurs after your amygdala, fear circuits, and fight or flight response is activated to prepare your body for survival, to either fight or run.
The problem with hypervigilance and hyperarousal that occurs in PTSD is that it is no longer adaptive, and is no longer helpful, as even when the danger is not present, you will have hypervigilance when it is not really needed.
To help you decrease your hypervigilance, you can learn more about how it develops when PTSD develops, and how CBT (cognitive behavioral therapy) can treat it, from the following video: