Complex sentence: Prince Salman's appointment as Defense Minister is seen as an important move, because it means that he is the next heir in line after Crown Prince Nayef bin Abdulaziz, and it enables him to experience a powerful govrnmental post before his future appointment as crown prince.
Split into simpler sentences: Prince Salman's appointment as Defense Minister is seen as an important move. This post is thought to enable him to experience a powerful govrnmental post before his future appointment as crown prince.