From The Art and Popular Culture Encyclopedia
Castration anxiety is an idea put forth by Sigmund Freud in his writings on the Oedipus complex; it posits a deep-seated fear or anxiety in boys and men said to originate during the phallic stage of sexual development. It asserts that boys, when seeing a girl's genitalia, will falsely assume that the girl had her penis removed, probably as punishment for some misbehavior. The boy then becomes anxious lest the same happen to him.
Castration Anxiety literally means the fear that one's penis will be cut off, but more profoundly it may symbolize the child's fear that he will, like Oedipus (see Oedipus Complex) lose his power (and his love object as well - ie. his mother). Thus the boy harbours an unconscious wish to kill the father.