God sent snakes to punish the people who had sinned by speaking against Him and Moses.
He then said to Moses to make a bronze snake. If any person who had been bitten by the snakes then looked at the bronze snake in obedience to God's command, they would live.
The people of Israel were at it again growing impatient and saying why have you Moses bought us into the desert to die. We do not like the food God supplies.
The people Moses was leading were again for getting very quickly the miracles God had performed for them. Even though the cloud by day and the fire by night was still with them.
The Bronze Snake
4 They traveled from Mount Hor along the route to the Red Sea, to go around Edom. But the people grew impatient on the way; 5 they spoke against God and against Moses, and said, "Why have you brought us up out of Egypt to die in the desert? There is no bread! There is no water! And we detest this miserable food!"
6 Then the LORD sent venomous snakes among them; they bit the people and many Israelites died. 7 The people came to Moses and said, "We sinned when we spoke against the LORD and against you. Pray that the LORD will take the snakes away from us." So Moses prayed for the people.
8 The LORD said to Moses, "Make a snake and put it up on a pole; anyone who is bitten can look at it and live." 9 So Moses made a bronze snake and put it up on a pole. Then when anyone was bitten by a snake and looked at the bronze snake, he lived.