The parochet (Hebrew: פרוכת) (also paroches; from the Aramaic parokta meaning "curtain" or "screen" ) is the curtain that covers the Aron Kodesh (Torah Ark) containing the Sifrei Torah (Torah scrolls) in a synagogue.
The parochet symbolizes the curtain that covered the Ark of the Covenant, based on Exodus 40:21. "He brought the ark into the Tabernacle and placed the screening dividing curtain so that it formed a protective covering before the Ark...".
In most synagogues, the parochet which is used all year round is replaced during the High Holy Days with a white one.
The term parochet is used in the Bible to describe the curtain that separated the Kodesh Hakodashim (Holy of Holies) from the main hall called "Hekhal"  of the Temple in Jerusalem. Its use in synagogues is a reference to the centrality of the Temple to Jewish worship.
Parochet of the Synagoge in Mühlhausen
Parochet in the Hurva Synagogue in Jerusalem
Parochet from 1797, Jewish Museum of Switzerland.