Because of the people's lack of faith to enter and capture the promised land, God forced them to wander in the desert for forty years.  While wandering, the people spoke out against God and against Moses, so God sent fiery serpents (venomous snakes) to bite them.  Many Israelites died.  Finally, the people confessed their sin and Moses prayed for the people.  God told Moses to make a brass snake and put it up on a pole.