One cause of anxiety. "neurosis" was freud's term for symptoms resulting from unconscious emotional conflict. Many such symptoms were identified. He believed that anxiety resulted when unacceptable unconscious material threatened to rise to consciousness. Nowadays we believe this is only one cause of anxiety. A variety of medical, as well as psychological, factors can lead to it as well.
Hides anxiety. Neuroses are "tricks" we play to hide anxiety. Someone who is "passive aggressive", say, finds his/her anger very anxiety-provoking (for childhood reasons). He/she therefore adopts a "what, me angry?" pose. That pose is habitual and neurotic; it masks the anxiety. Freud, channeling jackie mason, said that the goal is to "replace neurotic suffering with common human unhappiness" ;).