It's the curtain in front of the Holy Ark.
A couple of examples are the parochet (cloth curtain in front of the holy ark), which is decoratively embroidered; and the Torah scroll, which is adorned with a decorative crown, often made of silver.
I assume you mean the curtains on the Ark (known to Ashkenazi Jews - those from North and Eastern Europe - as the Aron Kodesh and to the Sephardi - Jews from Spain, Portugal and North Africa - as the Hekhal) used to contain the Sefer Torah, as any other curtains in a synagogue are likely to be practical and of little significance. These curtains are known as parochet (also spelled paroches or parokhet. Most Arks are also fitted with doors - in Ashkenazi and Mizrahi (Jews from the Middle East) synagogues, the curtains will be outside the doors while in Sephardi synagogues they'll be inside.
Some synagogue artifacts are: aaron kodesh-"holy ark" which contains Torah scrolls Torah scrolls- parchment scrolls on which the bible is written bimah-table where Torah scrolls are read ner tamid- "eternal light" is a lamb that reminds Jews of the temple in Jerusalem mechitzah- divider between male and female prayer areas Torah cover-cloth fabric covering the Torah scrolls when not in use yad- "hand" a metal, sticklike instrument used to mark one's place when reading the Torah parochet-curtain that covers the ark Elijah's chair-traditional chair reserved for Elijah the prophet. used only during brit mila