The Brazen Serpent was a bronze image of a serpent that was raised on a pole. The Israelites had been cursed by God with snakes due to their sin. Moses interceded for them, and God instructed Moses to construct the serpent. All who would look upon this serpent will be saved. Numb. 21:8, "Then the Lord said to Moses, 'Make a fiery serpent, and set it on a standard; and it shall come about, that everyone who is bitten, when he looks at it, he shall live.'"
The serpent represents Christ, who was lifted up on the cross, and all who look upon the cross by faith are saved. John 3:14-15, "And as Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up; 15 that whoever believes may in Him have eternal life."