5 And the people spoke against God and against Moses, "Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and we loathe this worthless food."
6 Then the LORD sent fiery serpents among the people, and they bit the people, so that many people of Israel died.
Once again the Israelites rebel, failing to trust God and speaking against Moses and God. This time God sends punishment in the form of 'fiery' serpents which cause many deaths. The serpents seem to be supernatural in nature (fiery) though it is not clear whether they resemble fire or merely have a fiery/painful bite.
The people regret their complaints and ask Moses to save them. God commands Moses to put an image of a fiery serpent on a pole and the people who are bitten must look to this to be spared death.
This image up on a pole is a symbolic representation of Christ who was up on the cross. As the people had to look to the bronze serpent to be saved then, we must look to Christ to be saved now. The New Testament speaks of this parallel in the book of John.