Acute Anxiety Disorder

A family member of mine was recently diagnosed with Acute Anxiety Disorder, and I'm not very sure of what it is. From my understanding it in't very serious at all, and my dad tried explaining it to me, but I'm not sure how much of what he is saying is accurate. I understand most mental conditions are very broad on what symptoms and effects can go with them, but I would just like a little more clarity. Thanks!