Means 'Commander of the Belivers' or 'Commander of the faithful'. This title was given to the Imam Ali (AS) specifically by the Prophet Muhammed (SAW), but was later attributed to anybody who became a khalif (not by the Prophet (SAW)), so there is amir al-mumineen Abu Bakr, amir al-mumineen Umar, Uthman, muawiyah, yazeed etc.