The High Priest is a unique position for Judaism. While the High Priest was a major religious leader, his most important function was to enter the Holy of Holies in the temple once a year on Yom Kippur to gain atonement for the sins of the people of Israel. Since the destruction of the last temple, this act has not been possible. But many within the modern nation of Israel are ready for when a new temple is built.
There are many requirements to be the High Priest:
- He must be male
- He must be a direct descendent of Aaron (brother of Moses)
- He must be widely recognized as the greatest priest in Israel
Of course, this leaves open the question of "Who decides?" In any time and any election, the question of who is "the greatest" is a matter of debate. Historically, there have been different answers to the question:
- God appoints the High Priest (as He did with Aaron)
- The King appoints the High Priest (as Solomon did with Zadok)
- The Sanhedrin appoints the High Priest (as they did for Ananias)
- The next High Priest may be the son of the last High Priest (as Aaron's sons were after him)
The Bible requires a high priest, but Israel has had to maintain the position thru many different historical contexts.
Information largely comes from here: http://www.chabad.org/library/article_cdo/aid/994923/jewish/How-Is-a-High-Priest-Selected.htm and http://en.wikipedia.org/wiki/High_Priest_of_Israel