Agoraphobia literally means “fear of squares” (or open spaces). Somebody who suffers from it is afraid of the unknown. People who are agoraphobic are, for example, afraid of:
Leaving their familiar and comfortable surroundings
In that case the fear is not necessarily of the open space or crowd itself, but rather of potentially having a panic attack in a place where you can’t get away or no help is available. Agoraphobia leads to the avoidance of crowds and public events. In extreme cases people no longer leave their homes. The symptoms of agoraphobia can resemble those of claustrophobia or a social phobia.