“Then the Lord said to Moses, “Make a snake image and mount it on a pole. When anyone who is bitten looks at it, he will recover.” So Moses made a bronze snake and mounted it on a pole. Whenever someone was bitten, and he looked at the bronze snake, he recovered.”
Numbers 21:8-9 HCSB
At first I wasn’t sure what to make of this, but as I read and thought on it, I realized that it isn’t an idol – the Lord was instead looking for obedience. He didn’t ask much – just that the people would look at the bronze snake and be healed. If they didn’t look on it, if they didn’t have the faith to just do that, they wouldn’t be healed.
This all came because the people were complaining about God and Moses, and because they had been hit with an additional hardship, so the Lord sent poisonous snakes among them.