Jamieson, Fausset and Brown on Numbers 21:7-9

7-9. the people came to Moses, and said, We have sinned--The severity of the scourge and the appalling extent of mortality brought them to a sense of sin, and through the intercessions of Moses, which they implored, they were miraculously healed. He was directed to make the figure of a serpent in brass, to be elevated on a pole or standard, that it might be seen at the extremities of the camp and that every bitten Israelite who looked to it might be healed. This peculiar method of cure was designed, in the first instance, to show that it was the efficacy of God's power and grace, not the effect of nature or art, and also that it might be a type of the power of faith in Christ to heal all who look to Him because of their sins (Joh 3:14, 15; see also on 2Ki 18:4).