Short answers, pulled from the story.
The Golden Fleece is the fleece of Chrysomallos, a golden-woolled winged ram that rescued Phrixus and carried him to Colchis. Phrixus sacrificed the ram there and gave the fleece to King Aeëtes, after which it became a symbol of authority and kingship. Jason and the Argonauts later stole it with the help of Medea, Aeëtes' daughter.
King Pelias ordered Jason to retrieve the Golden Fleece so that Jason could rightfully claim the throne of Iolcus in Thessaly. Bringing back the fleece was a political condition tied to Jason's legitimacy as a ruler, not simply a test of courage.
King Aeëtes hung the Golden Fleece on an oak tree in a grove sacred to Ares, the god of war. It was guarded by a dragon that never slept, whose teeth could be planted in the ground to grow into soldiers.
The Golden Fleece is a symbol of authority and kingship. Scholars have also connected it to placer gold mining in ancient Georgia, where sheep fleeces were used to collect gold flakes from streams, a practice Strabo described directly.
Apollonius of Rhodes composed the Argonautica in mid-third-century BC Alexandria. A second, lesser-known Argonautica was written in Latin by Valerius Flaccus during the reign of Vespasian.
Only two representations of the Golden Fleece on Attic-painted wares of the fifth century BC have been identified. One is a krater at the Metropolitan Museum of Art and the other is a kylix in the Vatican collections, painted by Douris around 480-470 BC, in which Jason is shown being disgorged from the dragon's mouth and the fleece hangs from an apple tree.