When God made Aaron, the brother of Moses, the first high priest, he officially set up the office of priest in perpetuity (a man so ordained would remain a priest forever and the office of the priesthood would continue forever):
Exodus 29:9 – 11 To wit, Aaron and his children, and thou shalt put mitres upon them; and they shall be priests to me by a perpetual ordinance.
The high priest was endowed with awesome responsibilities, special powers and extraordinary authority. A high priest normally served for life (until later times, when that was changed). He was in charge of all the other priests, as well as the administration of the Tabernacle or Temple. When he died, his office was to be handed down to his successor:
Exodus 29:29 – 30 And the holy vesture, which Aaron shall use, his sons shall have after him, that they may be anointed, and their hands consecrated in it. He of his sons that shall be appointed high priest in his stead, and that shall enter into the tabernacle of the testimony to minister in the sanctuary, shall wear it seven days.
It was also normal for the high priest to prophecy. His prophetic words were considered to come straight from God. This was true even for corrupt high priests (like Caiphus, who condemned Jesus):
John 11:49 – 53 But one of them, named Caiphas, being the high priest that year, said to them: You know nothing. Neither do you consider that it is expedient for you that one man should die for the people and that the whole nation perish not. And this he spoke not of himself: but being the high priest of that year, he prophesied that Jesus should die for the nation. And not only for the nation, but to gather together in one the children of God that were dispersed. From that day therefore they devised to put him to death.