We are all imperfect and do bad things which God calls sin. God is so good that by comparison, us doing even what we think is a small bad thing actually is so bad it deserves death as a punishment! But God doesn't want us to die because He loves us, so He sent His "son" Jesus (God in the flesh) to be the one perfect person who didn't deserve punishment to take our punishment for us. If we believe in Jesus, then we don't have to die forever and instead, after our earthly body dies, we will live forever in heaven with Him. That's the simplest way I can explain it. That one little verse is "all about" a lot!
John 3 :16 says "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." This means that God loves us so much that He gave the only son He has. Can you imagine that? Giving the only son you have and you know you are not going to have any more children, and you gave him up because you loved the whole world. To God, it didn't matter that we were sinners, He loved us so so much that He sent his Son. He sent his Son that who ever believes in Him, in the Father and Son shall not die, but have eternal life. It's not a simple concept; it's hard for the human mind. Hope this helps!
No other verse in the Bible so succinctly summarizes God's relationship with humanity and the way of salvation. Some consider John 3:16 as the "theme verse" for the entire Bible. John 3:16 tells us of the love God has for us and the extent of that love-so great that He sacrificed His only Son on our behalf. John 3:16 teaches us that anyone who believes in Jesus Christ, God's Son, will be saved. John 3:16 gives us the glorious hope of eternal life in heaven through the love of God and death of Jesus Christ.http://www.gotquestions.org/John-3-16.html
It is basically the Mission Statement for the whole bible as well for Christs/Gods purpose. God loves humanity so much He sent His son, Jesus, with the specific purpose of being the final ultimate sacrifice for us so that whoever believes in and accepts the Truth of this great and free gift will have eternal life in heaven after death on earth. Verse seventeen goes on to say God did not send Christ to condemn the world, He sent Christ so we could all be saved through him and his great sacrifice.