The Lishkas HaParvah in the Avodah of Yom Kippur- Who was הַפַּרְוָה? Why does the High Priest purify himself there?

The Lishkas HaParvah in the Avodah of Yom Kippur- Who was הַפַּרְוָה? Why does the High Priest purify himself there? — מובחר ועד הנגינה