Agoraphobia, meaning "fear of the marketplace" in Greek, is a type of anxiety disorder. It's a condition where a person feels intensely anxious about being trapped in certain situations (i.e., public places or places where crowds gather) from which they can't easily escape. The result is a feeling of panic.
Movie theatres, long lineups in banks or stores, and buses or subways are examples of problem places for people with agoraphobia. They often end up avoiding these situations or only go with someone they feel "safe" with. In extreme cases, they may end up housebound, terrified to go anywhere in public.
Agoraphobia affects between 0.5% and 1% of the population, but more women are affected than men.