This is the theme of this chapter–Jesus the Son of Man.
The Israelites had sinned before the Lord. God sent serpents in the camp to bite and kill them. They cried out to the Lord for help, and Moses prayed in their behalf. http://ref.ly/Nu21.9; http://ref.ly/Jn8.28. The serpent raised up was for the deliverance of disobedient Israel. When the Israelites looked up at the serpent, they were spared. When Jesus was raised up above the earth, it would bring deliverance for all humanity. Verse 15 explains it best.
John 3:15 (KJV)
(15) That whosoever believeth in him should not perish, but have eternal life.
Whoever has faith in Jesus Christ will not die according the law. They shall have eternal life. When a believer leaves this world by death, they are immediately in the presence of the Lord.