With agoraphobia you feel safe only when you are in certain environments.
Your anxiety increases every time your "safe place" is unavailable, blocked, or becomes more distant.
The thought of being in a dark cinema, away from the exits, is intolerable. You do everything possible to stay in your safe places, and every new environment presents the challenge to identify escape routes in case you begin to panic. Any travel you might do is restricted, and anxiety increases the further away from your safe place that you go.
If agoraphobia remains untreated, the geography of your safe place can shrink until you are confined, like a prisoner, to your home or bedroom. The underlying fear is of the anxiety itself — you're afraid that you will have to endure nausea, an adrenalin rush the likes of which you would wish on no one, a pounding heart, and the feeling of doom that predicts utter annihilation.