Through Moses, God made a way for the Israelites to find physical healing from the poisonous snakes. Jesus explains to Nicodemus how Moses lifting up the serpent is a picture of how God has made a way for us to find eternal life. Bro. Jeff explains in more detail here.
John 3:9-15 (KJV)
9 Nicodemus answered and said unto him, How can these things be?
10 Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?
11 Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
12 If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?
13 And no man hath ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.
14 And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up:
15 That whosoever believeth in him should not perish, but have eternal life.
4 And they journeyed from mount Hor by the way of the Red sea, to compass the land of Edom: and the soul of the people was much discouraged because of the way.
5 And the people spake against God, and against Moses, Wherefore have ye brought us up out of Egypt to die in the wilderness? for there is no bread, neither is there any water; and our soul loatheth this light bread.
6 And the Lord sent fiery serpents among the people, and they bit the people; and much people of Israel died.
7 Therefore the people came to Moses, and said, We have sinned, for we have spoken against the Lord, and against thee; pray unto the Lord, that he take away the serpents from us. And Moses prayed for the people.
8 And the Lord said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live.
9 And Moses made a serpent of brass, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of brass, he lived.
18 Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.
19 For as by one man’s disobedience many were made sinners, so by the obedience of one shall many be made righteous.
2 The Lord looked down from heaven upon the children of men, to see if there were any that did understand, and seek God.
3 They are all gone aside, they are all together become filthy: there is none that doeth good, no, not one.
20 For there is not a just man upon earth, that doeth good, and sinneth not.
6 All we like sheep have gone astray; we have turned every one to his own way; and the Lord hath laid on him the iniquity of us all.
6 But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.
22 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.
11 And I will punish the world for their evil, and the wicked for their iniquity; and I will cause the arrogancy of the proud to cease, and will lay low the haughtiness of the terrible.
2 Peter 2:9
9 The Lord knoweth how to deliver the godly out of temptations, and to reserve the unjust unto the day of judgment to be punished:
2 Thessalonians 1:7-9
7 And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels,
8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
9 Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;
12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
15 Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death.
8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
2 Timothy 1:10
10 But is now made manifest by the appearing of our Saviour Jesus Christ, who hath abolished death, and hath brought life and immortality to light through the gospel:
10 The thief cometh not, but for to steal, and to kill, and to destroy: I am come that they might have life, and that they might have it more abundantly.
24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.