Wiktionary
n. (context idiomatic US English) Figuratively, a prize or goal. Often used with respect to employment goals e.g. promotion, better job, etc.
WordNet
n. a rich opportunity or a prize; "he missed the brass ring at the Miami convention"
Wikipedia
A brass ring is a small grabbable ring that a dispenser presents to a carousel rider during the course of a ride. Usually there are a large number of iron rings and one brass one, or just a few. It takes some dexterity to grab a ring from the dispenser as the carousel rotates. The iron rings can be tossed at a target as an amusement. Typically, getting the brass ring gets the rider some sort of prize when presented to the operator. The prize often is a free repeat ride. The phrase to grab the brass ring is derived from this device.
Usage examples of "brass ring".
Standing directly beneath the brass globe, he jumped up and accurately hooked the brass ring with the key.
A brass ring in the animal's nose was tied to a wooden stick such that if it stopped running, its nose was pulled.
He stuck his forefinger through the brass ring and gently pulled the wire taut.
He sat on the edge of the pallet and held out a simple brass ring engraved with the seal of the King’.
He sat on the edge of the pallet and held out a simple brass ring engraved with the seal of the King's Eagles.