Complex sentence: The Rashidun Caliphate (') is the collective term comprising the first four caliphs--the "Rightly Guided" or Rashidun caliphs (')--in Islamic history and was founded after Muhammad's death in 632 (year 11 AH in the Islamic calendar).
Split into simpler sentences: The Rashidun Caliphate (') was the Islamic caliphate in the earliest period of Islam, comprising the first four caliphs--the "Rightly Guided" or Rashidun caliphs ('). It was founded after Muhammad's death in 632 (year 11 AH in the Islamic calendar).