In the OT, when Moses had led them out of Egypt and into the dessert, they started to complain about the manna, so God sent snakes that bit them and many died. When they cried out to Moses, God told him to put a snake on a pole and if they looked at it, they would be healed. Numbers 21:5-9 It was a symbol of the people’s sin displayed (serpents are a symbol of sin). It was a foreshadow of Christ being hung on the cross for our sins. Here is a commentary on it. https://www.blueletterbible.org/comm/smith_chuck/sermonnotes_num/num_17.cfm